This week's junior doctors' strikes will pose immediate "risks to patient safety", the head of the NHS Confederation has warned.
Matthew Taylor said the stoppage would also have a longer-term "catastrophic impact" on NHS waiting lists with the knock-on effect of "up to 350,000" cancelled operations and appointments.
Mr Taylor, whose organisation represents NHS managers, said the Acas conciliation service should be brought in to resolve the dispute between the government and junior doctors.
"It's going to be an incredibly tough week. We've got four days of industrial action which of course come after the Easter Bank Holiday weekend, followed by another weekend, so you're talking about 10 or 11 days when the NHS is not able to operate at full strength," Mr Taylor told Sky News.
He added: "These strikes are going to have a catastrophic impact on the capacity of the NHS to recover services.
"The health service has to meet high levels of demand at the same time as making inroads into that huge backlog that built up before Covid, but then built up much more during Covid.
"That's a tough thing to do at the best of times - it's impossible to do when strikes are continuing."
Junior doctors will walk out from 6.59am on Tuesday until 6.59am on Saturday.
The British Medical Association wants the government to restore junior doctors' pay to the equivalent of 2008 levels, following years of real terms cuts in with their wages, which have fallen by more than a quarter.
Health Secretary Steve Barclay says the demand is "unrealistic".
Speaking on Monday morning the NHS Confederation chief executive Mr Taylor said the dispute was currently "a battle of rhetoric rather than talks".
"It's depressing that there seems to be no movement at all from the two sides of this dispute over the last few days," he said.
"We should consider asking the Government and the trade unions to call in Acas, the conciliation service, to provide some basis for negotiations, because if anything the positions seem to have hardened over the last couple of days."
Asked whether everyone who needs urgent care this week will get it, he replied: "We hope so. There's no point hiding the fact that there will be risks to patients - risks to patient safety, risks to patient dignity - as we're not able to provide the kind of care that we want to."
Mr Taylor said there was "no question" that the stoppage would be more disruptive than a previous 72-hour walkout last month.
Last week the co-chairs of the BMA junior doctor committee Dr Vivek Trivedi and Dr Robert Laurenson said the strike could still be called off if the government made a "serious and credible offer" to reverse the pay cuts.
"We very much expect that an offer will be a starting point in the discussions, rather than the final deal we agree, but until the Minister's office makes that offer and agrees to meet with us, we cannot consider stopping the strike action and starting negotiations," they said.
