Protections for state pensions, the NHS and defence spending cannot be guaranteed if Britain leaves the EU, David Cameron has warned.
The Prime Minister said the strain on the public finances caused by Brexit would threaten the end of the "triple lock" guarantee for OAP payments and the ring-fencing of health funding.
And Chancellor George Osborne said the armed forces could see their budgets slashed by £1-£1.5 billion a year as the wider economy shrank.
The Remain camp sought to press home its central message of likely economic turmoil in the event of a vote to quit the bloc on June 23 amid signs of a hardening of public opinion against the EU.
Vote Leave downplayed the accuracy of a poll giving it a 10-point lead though many of its campaigners seized on the survey as proof the momentum was firmly towards divorce from Brussels.
The top Tory pair insisted that they remained committed to manifesto promises to protect funding in the three areas.
But Mr Cameron told the Observer: "If we leave the EU, independent and respected experts like the IFS and the NIESR say that by 2020 we will face a black hole in our public finances of up to £40 billion.
"In those circumstances, future funding for the NHS could be at risk. Our ability to ring-fence and protect spending on health could be at risk too.
"This is the cold reality of leaving the EU - that's why doctors, nurses and the boss of the NHS all say we will be stronger, safer and better-off in the EU."
Writing in the Sunday Telegraph, the Prime Minister said ministers would be "forced" to re-examine pensioner benefits.
"We've made a special effort to protect pensioners," he said - highlighting the "triple lock" which says the state pension will rise in line with earnings, inflation or by 2.5%, whichever is higher, and the protection of bus passes and free TV licences.
"We did all this in the expectation of a growing economy. But if we had a big black hole, we could struggle to justify this special protection any longer.
"In fact, even if we could justify it morally, it wouldn't actually be affordable. Not when pensions represent a huge portion of public spending - over £90 billion this year - and when inflation is forecast to hit 4% if we leave Europe.
"So here is the reality: if we leave, the pensioner benefits would be under threat, and the Triple Lock could no longer be guaranteed in the long term."
Mr Osborne said Brexit would mean "a new dose of austerity, more years of public spending cuts".
"If you look at the numbers, the defence budget would have to be cut by between one and one and a half billion pounds," he told the Sun on Sunday.
"It's the last thing I want to do because I want the country to stay in the European Union, but if we leave the European Union Britain is smaller and so Britain's armed forces will be smaller and that means fewer planes and ships and personnel to defend us.
"So it is both a hit to our national economic security but also our national security.
"Even if you could keep to your Nato commitment of 2% of GDP, GDP would be smaller so 2% would be smaller."
Current plans to procure the latest hardware, ships and aircraft "would be up in the sky again because the country would be facing a big hole in the public finances", he said.
Pro-Brexit ex-Cabinet minister Iain Duncan Smith accused his party leader of a "vindictive and desperate attempt to bully and frighten the British people".
"This is a baseless threat," he said.
"Experts have shown that leaving the EU - and introducing an Australian-style points system - would lead to a higher state pension.
"What we now have is a vindictive and desperate attempt to bully and frighten the British people - particularly pensioners - all in a frantic bid to rescue a failing campaign.
"The truth is that these are policy choices and the Conservative manifesto said that protecting pensioners was a priority. It is now apparent that there is nothing they will not use or jettison in their efforts to keep us in the European Union."
The Remain campaign has been boosted by support from the Archbishop of Canterbury who said Britain should be "a country for the world" and warned against "succumbing to our worst instincts" over immigration.
Writing in the Mail on Sunday, the head of the Church of England, the Most Rev Justin Welby, said he would vote to stay in the EU to avert economic damage that could harm the poorest.
"We each have to make up our own minds," he said. "But for my part, based on what I have said and on what I have experienced I shall vote to remain."