Sir John Major has launched an attack on senior Conservatives in the Vote Leave campaign, accusing them of exaggerating arguments to exit the EU.
The ex-Tory PM said claims by Michael Gove, Boris Johnson and Iain Duncan Smith did not bear factual scrutiny and warned about the tone of some exit campaigners' rhetoric on immigration.
Vote Leave said Sir John was "wrong".
MP Peter Lilley said Leave had got the tone right on immigration and suggested Sir John was "inventing a straw man".
"He was saying there is misuse of the issue - and that certainly would be wrong - but he didn't give any examples of this misuse," he told the BBC.
Pro-exit employment minister Priti Patel said Sir John was a "respected former prime minister" but it would be the public - not politicians - who would decide the outcome of the referendum.
The UK votes on whether to stay in or leave the European Union on 23 June.
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In a hard-hitting speech in Oxford, Sir John - whose seven years in Number 10 were dominated by internal party rows over Europe - said the Remain side must "debunk myths" about immigration, sovereignty and the economy to prevail in the referendum.
Economic facts were simply "brushed aside" by Vote Leave, he said, adding that those who favoured an EU exit were propagating "absurd falsehoods".
Why this issue matters What the leave and remain sides are saying about the cost of membership in the #EUref campaignThe debate
- The UK is a net contributor to the EU budget
- The gross contribution in 2015 was £17.8bn but the UK rebate was worth £4.9bn
- £4.4bn was also paid back to the UK government for farm subsidies and other programmes
Leave
- The gross cost works out at £350m a week
- If the UK left, billions of pounds would become available for other priorities
- The UK would also be able to decide how to spend the money that the EU transfers back to it
Remain
- Economic benefits of EU membership easily outweigh the cost
- Other countries contribute more per person than the UK does
- After Brexit, the UK would still have to contribute to the EU budget to retain access to the single market
"There is no shortage of their exaggerations," said Sir John, citing the cost of being in Europe as a "clear example".
"(UKIP's) Nigel Farage, Iain Duncan Smith and Boris Johnson all put it at £20bn a year - it's even on Boris' battle bus; Michael Gove is more modest at £18bn (£350m a week), all of which, they tell us - if only we could be free of Europe - would be spent on the health service and our hospitals.
"If only," he said - adding that their figures are "wrong".
Voting to leave the EU was not "taking back control", as Leave campaigners argue, Sir John said: "It is nothing other than reckless, imprudent folly."
'Pure demagoguery'
Sir John also dismissed arguments made by Leave campaigners, including Mr Johnson, that Britain would regain its sovereignty if it exited the EU.
"We should not forget that - in well over 90% of the votes cast in Brussels - the UK wins," he said.
"The caricature that we are repeatedly voted down in Europe is ill-informed nonsense."
On immigration, Sir John suggested some Leave campaigners were in danger of morphing in to UKIP by pushing the issue and warned it risked creating "long-term divisions" in society.
He singled out for criticism claims made by Leave campaigners - including Mr Gove - that EU expansion would open Britain's borders up to another 88 million people.
Image copyright Getty Images"I assume this distortion of reality was intended to lead the British people into believing that almost the entire population of possible new entrants will wish to relocate to the UK.
"If so, this is pure demagoguery. I hope that - when the heat of the referendum is behind us - the proponents of such mischief making will be embarrassed and ashamed at how they have misused this issue," he said.
Why this issue matters What the leave and remain sides are saying about immigration in the #EUref campaignThe debate
- Total net migration to the UK is running at over 300,000 a year despite the government’s target of cutting it to under 100,000
- Migration from the EU accounts for just under half the total
- EU citizens have the right to live and work in any member state
Leave
- It is impossible to control immigration as a member of the EU
- Public services are under strain because of the number of migrants
- High immigration has driven down wages for British workers
- The official figures underestimate the true level of migration
Remain
- Immigrants, especially those from the EU, pay more in taxes than they take out
- Cameron's EU deal means in-work benefits for new EU migrant workers will be limited for the first four years
- Outside the EU the UK would still have to accept free movement to gain full access to the single market
- Immigration is good for the economy
Sir John said he did not want to "silence" debate on immigration, as it was a "legitimate" issue - but he warned that "care honesty and balance" was needed to avoid "raising fears or fuelling prejudice".
Defence minister and Vote Leave campaigner Penny Mordaunt said that while she had "great respect" for Sir John, "he's wrong" on immigration.
"We've got to talk about immigration. It's of huge concern to the public," she said, saying the UK "does not have full control of our borders".
"We can't keep certain people out, we can't deport certain people. That's a real problem," she added.
Image copyright Getty ImagesMeanwhile, Conservative David Davis said it was "grossly unfair" to dismiss concerns about immigration as "UKIP".
"Sir John is right to warn that debates on immigration must not slip into xenophobia. But there are also great dangers from ignoring the very real concerns of the voters, and dismissing them as divisive and prejudiced," he said.
He added that there were "even greater dangers" from letting immigration "run completely out of control".