A "level four" national emergency is declared when the hot weather is so extreme that "illness and death may occur among the fit and healthy" as well as vulnerable people, the UK Health Security Agency states.

If implemented, it could cause widespread disruption to schools, travel, health services and even nuclear power plants.

Senior officials held a crisis COBRA meeting inside Downing Street on Monday to consider the move, as forecasters said there is a 30% chance that Sunday will be the hottest day the UK has ever seen.

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There is a possibility that local hosepipe restrictions are introduced, especially in the south, if high temperatures persist.

It comes after the Met Office issued an amber weather warning for extreme heat across parts of the UK on Sunday, as temperatures are expected to soar into the 30s.

The rare warning means there could be a danger to life or potential serious illness as a result of the weather.

It is being enforced across the East Midlands, East of England, London, South East, North East, North West, South West, Wales, West Midlands and Yorkshire and Humber.

"At the moment, we are looking at a 30% chance of seeing the highest temperature recorded in the UK, if that does happen we will have to record higher than 38.7C - which was back in 2019 in Cambridge," said Met Office forecaster Simon Partridge.

"Technically, if we don't drop below 20C overnight that is classed as a tropical night, so parts of Yorkshire - which sounds ridiculous - are going to have a tropical night tonight," he added.

Rail delays, advice for drivers and deployment of gritters

However, the sunshine could cause some issues such as road closures and delays or cancellations to rail and air travel.

Network Rail has warned speed restrictions are likely to be in place this week on the worst-affected parts of the network.

For those driving, the RAC has recommended carrying water in the car and starting journeys earlier in the day.

Fire services have also urged people to have picnics instead of barbecues during the warm weather after a spate of grass fires.

In Hampshire, the council has said it is preparing to deploy gritters to protect cars against melting roads.

In Lincolnshire, the high temperatures have caused a key RAF base to halt flights after the tarmac melted in the sunshine.

Some parts of the UK have already seen record temperatures, with Wales recording its hottest day of the year so far after the mercury reached 28.7C in Cardiff's Bute Park on Monday.

On the same day, temperatures hit the low 30s in England, with Northolt, west London, recording 32C.

Railway lines on Battersea Bridge, in central London, caught fire as a spark ignited wooden beams after speed restrictions were introduced to stop rail lines from buckling in the heat.

Elsewhere, Aboyne in Aberdeenshire, Scotland, basked in 27.5C and it was 24.2C in Armagh, Northern Ireland.

What is the weather looking like for the rest of the week?

The Met Office says the mercury is predicted to rise to 30C in London, Chelmsford, in Essex, and Sudbury, in Suffolk, on Tuesday while the rest of England is expected to see temperatures soar to the late 20s.

The whole of England is largely expected to experience sunny days with some cloud over the next week to match the temperatures of mid to high 20s.

It will cool off a little on Friday, with the north-west and north-east of England predicted to have some rain with temperatures reaching highs of 19C.

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Temperatures are expected to spike to 32C in Reading and London on Sunday and in excess of 35C in the southeast.

Scotland and Northern Ireland meanwhile are predicted to have a cloudier week, with rain forecast for Tuesday and Friday.

Weather forecasting models suggest it is possible - though not likely - temperatures could reach 40C in the UK next weekend.

It would mark the first time the temperature has ever been recorded in the country.