The stoppages are the result of the failure to reach an agreement over pay, jobs and conditions.

Here's what you need to know - and how disruptions may affect your journey.

When?

The rail strikes are the biggest in decades.

The first day of industrial action took place on Tuesday, with further strikes taking place on Thursday 23 and Saturday 25 June in a dispute over pay and redundancies.

The RMT and Unite unions also took part in industrial action that affected the London Underground on Tuesday.

Network Rail has warned that the strikes will cause six days of disruption because of the knock-on effect on services on the days in between.

Why are they happening?

The RMT general secretary Mick Lynch said thousands of jobs were being cut across the rail networks and workers were facing below-inflation pay rises.

He said: "It has to be restated that the source of these disputes is the decision by the Tory government to cut £4bn of funding from our transport systems - £2bn from National Rail and £2bn from Transport for London."

What does the government say?

The government has urged the unions to scrap the action.

A Department for Transport spokesperson said: "Strikes should always be the last resort, not the first, so it is hugely disappointing and premature that the RMT is going ahead with industrial action.

"The government committed £16bn - to keep our railways running throughout the pandemic while ensuring not a single worker lost their job.

"The railway is still on life support, with passenger numbers 25% down and anything that drives away even more of them risks killing services and jobs."

Will any train services be running?

The action by tens of thousands of rail workers will affect millions of commuters and cripple services for most of the week.

Only around half of Britain's rail network will be open on strike days, with around 20% of services running on lines, according to Network Rail.

Lines will only be open from 7.30am to 6.30pm, meaning trains will start later and finish earlier than usual.

On days between the strikes - 22 and 26 June - 60% of services will run.

A special timetable for 20 to 26 June has been published.

Can I get a refund?

"If your service has been cancelled, delayed or rescheduled, you will be entitled to a change or refund from the original retailer of your ticket," Network Rail says - but adds that an admin fee of "no more than £10 may apply".

You are entitled to a full refund if your train was cancelled - it does not matter what type of ticket you bought, according to Citizens Advice.

The amount you can claim for a delay depends on the type of ticket you bought.

Most rail companies are part of a scheme called Delay Repay, which will legally entitle you to 25% of the cost of a single journey if you are delayed by 15 to 29 minutes.

You will be eligible for 50% of your ticket price if you get to your destination between 30 minutes and an hour late, and a full refund if you arrive more than an hour late.

The government said season ticket holders will be able to claim full refunds on strike days - a more generous scheme than usual.

Can I change my ticket times?

If you have a ticket for travel on any of the strike days (21, 23 or 25 June) you can use this ticket either on the day before the date on the ticket or through and including the Monday and Tuesday of the following week, Network Rail says. This does not apply to season tickets.

Check with your travel provider as they may allow you to use your ticket on another company or an alternative route.

Here is a breakdown of what is happening on different lines this week.

Transport for London

Though the industrial action affecting the London Underground has already taken place this week, Transport for London has warned national rail strikes are expected to impact services on the Elizabeth line and London Overground services, and some Tube services, on strike days and until mid-morning on the days after the walkouts.

There will be a reduced service on the London Overground and Elizabeth line (where TfL uses national rail assets) as well as parts of the Tube on strike days.

London Overground services will run a reduced service from 7.30am on strike days and customers are advised to complete journeys by 6pm.

Disruption is likely to follow the strikes on parts of the Tube, Elizabeth line and London Overground on Friday 24 and Sunday 26 June, TfL said.

Avanti West Coast

A "significantly reduced timetable" will be in place on the strike days. Services from London Euston will run every hour to Manchester, Liverpool, Birmingham and Preston.

North Wales, Shrewsbury, Blackpool and Edinburgh have no Avanti West Coast services on strike days.

Trains will not be calling at Stockport, Macclesfield, Stoke-on-Trent or Runcorn, and these stations will be closed.

Customers with existing tickets for travel from Tuesday to Sunday can claim a full refund with further sales suspended to "help reduce disruption and overcrowding".

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It will operate less than a third of normal services between 7.30am and 6.30pm.

This will include two trains per hour from London Fenchurch Street to Shoeburyness via Laindon, and the same frequency from London Fenchurch Street to Pitsea via Rainham. No trains will run via Ockendon or Chafford Hundred.

East Midlands Railway

Services will operate between 7.30am and 6.30pm, with no direct trains between Luton and London St Pancras on Saturday and Sunday. Just one train per hour will run in each direction on most routes.

West Midlands Railway

A number of services will not be running during the week. Strike days will see a "very limited service", as will Wednesday and Friday.

Great Northern

No more than two trains per hour will run between King's Cross to Ely, Cambridge, Peterborough, Welwyn Garden City and Stevenage via Hertford North. On strike days, the last trains will finish in the afternoon.

London North Eastern Railway

Around 38% of its usual trains will be running, which are likely to be very busy. The last train from London King's Cross to Edinburgh is at 2pm, while the final service to Leeds will depart at 3.05pm. The last train from Edinburgh to London is at 12.30pm, with the final departure from Leeds at 3.45pm.

London Northwestern Railway

Just two trains per hour between London Euston and Northampton, and one per hour between Birmingham New Street and Northampton. No trains will operate between London Euston and Crewe.

Great Western Railway

A number of services will not be running on strike days, including all those in Cornwall and Devon and on South Wales main line, Heart of Wessex line, Severn Beach line, North Cotswolds line and South Cotswolds line.

On Saturday, Greenford branch line services will not run.

There will be an "extremely limited service" on other routes, which will operate between 7.30am and 6.30pm.

Greater Anglia

A much reduced service on London routes will be running, with no regional or branch lines trains and no rail replacement buses. No more than two trains per hour will run into the capital, starting from 7.30am and finishing by 6.30pm.

Northern

Services will not be operating on most routes, with a "very limited" number of trains on the few running lines, including those from Leeds.

South Western Railway

A "severely limited service" will run between 7.15am and 6.30pm on strike days, and only on some routes. This includes only four trains per hour between London Waterloo and Woking, and two per hour between London Waterloo and Basingstoke.

Southeastern

Most stations and routes will be closed and a "severely reduced" service will run on strike days. Where trains are not running, people will be unable to travel.

Southern

Many stations and routes will be closed, with just two trains running per hour from London Bridge and London Victoria to southeast London and the coast. Services will run on the Brighton Mainline to London Bridge and London Victoria, with additional trains from Tattenham Corner, Epsom Downs, Sutton and West Croydon, via Crystal Palace.

Gatwick Express

No service on strike days. A Sunday service will run on the days after the strikes, beginning after 7.15am and finishing early.

Heathrow Express

Services will be "significantly affected" during the week, with a half hourly service operating between 7.30am and 6.30pm on strike days.

Stansted Express

A reduced frequency will be in place, with later first trains and earlier last trains. There will be two trains an hour, with no services running from Stansted Airport to Norwich and Cambridge.

Thameslink

Generally two trains per hour will run both north and south. There will be far fewer trains than normal on strike days, with nothing running between London St Pancras and London Bridge.

TransPennine Express

Most services will see a "significant reduction" of trains. Yarm, Scarborough, Seamer, Malton, Selby, Brough and Hull stations will be completely closed with no services calling there on strike days. There will also be significant disruption on Wednesday and Friday.

ScotRail

No trains will run north of Glasgow or Edinburgh on strike days. Just two trains per hour will run between the cities via Falkirk. They will operate between 7.30am and 6.30pm.

CrossCountry

No services will run from Birmingham New Street to Bristol Temple Meads, Cardiff Central, Peterborough, Cambridge or Stansted Airport across the three days.

A "very limited service" is to run between Bristol Parkway and Plymouth and Birmingham New Street and Newcastle and Edinburgh Waverley.

Just one train per hour is to run between Birmingham New Street and Manchester Piccadilly, Leicester, Leeds and York and Reading. It warned of the possibility of last minute changes to the timetable.

Lumo

There will be "some disruption" to services throughout the week, but as many as possible will run.

Merseyrail

There will be no train services and no rail replacement buses on strike days.

Chiltern Railways

Across the three days no trains will run north of Banbury, to Oxford, or to Aylesbury via Amersham. One train an hour each way will run to Banbury, Aylesbury via High Wycombe and between Aylesbury and Amersham.

The service will start from around 7.30am, with morning trains not arriving in London until after 9am. The final train from the capital will leave at 4.45pm.

Caledonian Sleeper

All departures are cancelled between Monday and Friday.

Grand Central

In most cases just three trains in each direction will be running on strike days. Although there is a full timetable planned on the other days, some services have been cancelled or will start later.

Transport for Wales

On Thursday a reduced service will run between Radyr and Treherbert, Aberdare and Merthyr Tydfil, with replacement bus services between Radyr and Cardiff Central.

There will be a reduced service between Radyr and Treherbert, Aberdare and Pontypridd, with replacement buses in operation between Radyr and Cardiff Central on Saturday. All other services will be suspended.

Hull Trains

On strike days, trains will only run between Doncaster and London King's Cross. Services on either side of the strike days will also be affected.

It has announced that a planned strike on Sunday 26 June has been called off after "reaching an agreement" with Aslef, adding that talks are continuing.