A four-day strike by junior doctors in the coming days will cause "unparalleled levels of disruption" as up to 250,000 appointments and operations could be delayed.

The walkouts, which are part of a long-running pay dispute, begin at 6.59am on Tuesday and last until 6.59am on Saturday 15 April.

NHS national medical director Professor Sir Stephen Powis said: "This next round of strikes will see unparalleled levels of disruption, and we are very concerned about the potential severity of impact on patients and services across the country.

"This time the action immediately follows a four-day bank holiday weekend, which is already difficult as many staff are taking much-needed holiday, and it will be more extensive than ever before with hospitals facing nearly 100 hours without up to half of the NHS medical workforce."

Writing in the Daily Telegraph health secretary Steve Barclay accused junior doctors of timing their strike to cause "maximum disruption" and that there is a "considerable risk to patient safety".

He also accused their union of taking a "militant stance" to force a 35% pay rise - which he said would add up to £20,000 a year to pay packets.

The NHS will prioritise emergency, critical and neonatal care, as well as maternity and trauma services, but it is still expected that many thousands of appointments - including for cancer care - will have to be postponed.

The British Medical Association (BMA) wants the health secretary to negotiate to resolve 15 years of "pay erosion", insisting that junior doctors have lost more than 25% of their pay in real terms.

The organisation has said the strikes could be avoided if the government makes a "credible" pay offer.

But the Department for Health and Social Care wants the strikes cancelled before it will enter into negotiations.

'We're happy to meet at any time'

Dr Mike Greenhalgh, deputy co-chair of the BMA's junior doctors' committee, told BBC One's Breakfast show: "We're happy to meet at any time.

"We would still meet him over the bank holiday weekend before the industrial action next week. And if he was to bring a credible offer to us, it could still, even at this late stage, avert action."

He apologised to patients having operations and appointment cancelled but added: "Patient safety was maintained at the last strikes, and it will be in these strikes."

'More concerned about this than about any other strike'

Dr Layla McCay, director of policy at the NHS Confederation, told BBC Radio 4's Today Programme: "In the last junior doctors' strike we saw about 175,000 appointments and operations having to be postponed.

"In terms of the disruption that we're anticipating this time, we reckon it could be up to about a quarter of a million so that is a huge amount of impact for patients up and down the country."

She added: "What we're hearing from our members who are health leaders across the whole system is that they are more concerned about this than they have been about about any other strike.

"They think that the impact is going to be so significant that this one is likely to have impact on patient safety and that is a huge concern for every healthcare leader."