The head of the NHS has issued a stark warning that Brexit would be “very dangerous” for the health service’s finances, staff levels and drug prices.
In one of the most important interventions so far in the EU debate, Stevens even hinted that lives could be at risk if the UK voted to leave in the referendum on June 23.
He told the BBC’s Andrew Marr that there would be an exodus of EU-born doctors and nurses - and singled out Brexit minister Penny Mordaunt’s local hospital as an example of the crisis the NHS would face.
Asked directly if lives would be put at risk, he replied: “It's been true for the 68 years of the NHS' history that when the British economy sneezes the NHS catches a cold.
“This would be a terrible moment for that to happen at precisely the time the NHS is going to need that extra investment."
With both Remain and Leave campaigns keen to cite the impact on the NHS in their leaflets, Stevens said it was “perfectly reasonable for the NHS itself, when asked, to lay out the balance of the advantage in a objective, non-sensationalist way”.
Stevens, whose remarks were seized on by David Cameron as proof of the strength of the ‘Remain’ campaign, said that lower economic growth from a Brexit would directly hit the health service.
"When Mark Carney says that the risk of a slowdown in economic growth, possibly a recession if we end up exiting the EU, if Mark Carney is right then that is a severe concern for the NHS because it would be very dangerous if at precisely the moment the NHS is going to need extra funding at that moment the economy goes into a tailspin and that funding is not there," he said.
Stevens pointed out that the NHS has “benefitted enormously” from EU doctors and nurses, and rounded on Defence Minister Penny Mordaunt, who also appeared on the programme, for suggesting Brexit would help the service.
“Take an example, Penny Mordaunt’s local hospital. I was looking at the figures as she was talking.
"In her local hospital, 80 of the doctors are from the rest of the European Union, 350 nurses in her local hospital are from the European Union. If only a proportion of those chose to up sticks and off on the 24th of June, that will create real problems in hospitals across the country.
“If there’s uncertainty about work permits, about migration regimes, passports and all the rest of it….
"We’ve got about 130,000 European Union nurses, doctors, care workers in the NHS and in care homes. And we should surely miss the benefit they bring were we to chose to leave.”
He added: “It is pretty clear that the balance of the advantage is such that the risks would be greater were we to find ourselves in an economic downturn or were we to find a number of our nurses and doctors contemplating leaving.
"And indeed, if the pound were affected because a lot of the drug treatments we buy are priced in euros and dollars, so that could make it more difficult for us to get the treatments we need at an affordable price.”
Stevens dismissed claims from Vote Leave that they would save £350m a week currently sent to Brussels and spend it on the NHS instead.
"Unfortunately the UK Statistics Authority has shown that that isn't actually money that would be available for the NHS because when you net off what we give with what you receive it's a rather lower figure than that.
"That lower figure, even if all of it were deployed to the NHS is enough to fund the NHS for 19 days each years...the other 11 and a half months of the year it's the performance of the economy that will count.”
And he ridiculed claims that the EU immigration was having a negative impact on the service, saying the UK had coped with large increases in population in the past, and pointing out how much migrants paid in taxes to fund healthcare.
“When the NHS was set up in 1948 we had a population of 50 million, we are at 65 million now so the NHS has perfectly successfully coped with a 15 million expansion in our population. Provided it is properly resourced from the proceeds of economic growth, it can do that.”
Under NHS guidelines on political neutrality, policy states that ‘No activity should be undertaken which could be considered politically controversial or influential, which could compete for public attention or which could be identified with a … designated campaign group’.
But Stevens said he was not advocating one campaign or another, stating "everybody else has got an opinion about the NHS".
"These are just the practical realities the NHS would be facing and as one of the nation's most important social institutions, it seems reasonable that as we take one of these important national decisions, this goes into the balance.”
On ITV’s Peston on Sunday, the Prime Minister said the country "wants to hear" from people like Mr Carney and Mr Stevens, and added that the NHS chief was "making a very important point".
But former foreign secretary Lord Owen told the BBC's Sunday Politics: "Simon Stevens is the manager of the NHS which is currently £3bn in debt.
"This man has presided now for a sufficient time to judge his management skills. In almost every part of the National Health Service there is an acute crisis."
Labour’s Shadow Health Secretary Heidi Alexander said Stevens was “absolutely right” in warning Brexit would prompt "a crisis that could leave us with fewer doctors, fewer nurses and frontline services at risk of closure.”
Writing in the Sunday Times today, former NHS chief executives Sir Nigel Crisp and Sir David Nicholson say Brexit would amount to a "leap into the dark" for the economy and would undoubtedly affect patients.