Senior doctors are walking out on 19 and 20 September, as well as 2, 3 and 4 October. Meanwhile junior doctors, who have staged 19 days of strike action since March, will also strike on 20, 21 and 22 September and 2, 3 and 4 October.

A health chief said the NHS is in "uncharted territory" due to the strikes, with thousands of patient appointments expected to be cancelled.

Saffron Cordery, deputy chief executive of NHS Providers, said this week's strike action "can't become the status quo".

When are the strikes?

Consultants are walking out on 19 and 20 September, as well as the 2, 3, and 4 October.

Junior doctors will strike on 20, 21 and 22 September and the 2, 3 and 4 October.

This means that Wednesday's strike this week, plus the three days next month, will see both consultants and junior doctors absent from work on the same days.

Why are junior doctors striking?

The strikes are primarily over pay.

The government has implemented a 6% pay rise for consultants and 6% plus a lump sum of £1,250 for junior doctors and has said there will be no further offers.

The British Medical Association (BMA), meanwhile, is calling for "full pay restoration" back to 2008/09 levels, saying pay has been eroded over several years.

The BMA said the wage for junior doctors has fallen 26% in between 2008 and 2022, with newly qualified medics making less than a barista in a coffee shop.

It has demanded a 35% pay rise for junior doctors to bring salaries back to 2008-2009 levels, calling this "pay restoration".

"The lack of investment in wages by the government has made it harder to recruit and retain junior doctors," the BMA has previously said.

"If junior doctors are forced out of the NHS because of poor pay and conditions, the services we all rely on to look after our loved ones will suffer."

On the picket lines of the March strike, junior doctors told Sky News why they were striking and described having to borrow money off family for medical exams, watching colleagues leave for better paid jobs abroad and how they were struggling to pay rent.

What impact is this going to have on the NHS?

Thousands of NHS appointments and operations could be delayed.

The service is expected to see a Christmas Day level of staffing when both consultants and junior doctors are off, with emergency care taking priority.

Ms Cordery said "it's all hands on deck in trusts across the country".

"This 'double whammy' of the first ever joint strikes by senior and junior doctors is the toughest test yet for trust leaders, ramping up pressure on already stretched services," she said.

"Ten months of industrial action have seen almost one million routine appointments and procedures delayed."

Earlier this week, NHS Providers said it had heard from one trust that it was having to reschedule care for more seriously ill patients who previously may have been protected from strike disruption.

Another trust is rescheduling appointments for patients already hit by strike action, while a third said it would deploy volunteers to support patients with meals and drinks as well as providing company in A&E departments during the strikes.

NHS Providers said the strikes are estimated to have cost the health service at least £1bn so far and trust leaders are worried that senior doctors are increasingly unwilling to cover shifts when junior doctors are striking.

Why will this strike be particularly bad?

Wednesday marks the first time both consultants and junior doctors will walk out at the same time, leading to unprecedented pressure on the NHS.

Both junior and senior doctors will also be absent from work on the same days next month.

"Even when consultants go back to work on Thursday, junior doctors will be on strike until Friday with more strikes by both groups and radiographers planned for early October," Ms Cordery said.

The October strike dates also coincide with the Conservative Party annual conference in Manchester.

Read more:
Emergency care to be prioritised
'Huge concern' over worst NHS strike yet
Share how this week's strike action is impacting you with Sky News

Is this the first time doctors have gone on strike?

No - junior doctors who are members of the BMA have held a total of 19 strike days since March.

They first walked out for 72 hours from 13-15 March. During that strike, about 175,000 appointments and operations were postponed.

This week's strike will be their sixth walk out.

Meanwhile, consultants previously walked out for 48 hours in July.

What does 'junior doctor' mean?

A junior doctor is a qualified doctor who has graduated from medical school and is on a training pathway to become a specialist or a GP, according to the BMA.

Full-time training can take between five and 11 years - more if it is done part-time.

Junior doctors make up around 45% of the NHS's medical workforce and two-thirds of them are members of the BMA.

What has the government said about the strikes?

The prime minister's official spokesman said on Monday: "These co-ordinated strikes will pose a huge challenge for the NHS and for patients, who will see their care significantly disrupted."

Meanwhile, Health Secretary Steve Barclay was open to discussions about the "non-pay elements" of the BMA's concerns but there were no plans to "revisit" the pay deal, the spokesman said.