The Brexit camp could be handed accidental victory if millions of young voters missing from the electoral roll fail to register, ex-Labour leader Ed Miliband is warning.
Mr Miliband is issuing a "call to arms" aimed at pro-European voters who only have until June 7 to register for the right to take part in the first EU referendum for more than 40 years.
The intervention came as Tory in-fighting escalated with pro-Brexit employment minister Priti Patel launching a scathing attack on the "chaos" of Britain's immigration controls, as she ridiculed economic predictions from Government colleagues in the Treasury.
Ex-Labour leader Mr Miliband warned that 1.5 million of the six million 18-24-year-olds eligible to vote are not registered, nor are a quarter of the eight million 25-35-year-olds.
"Today is a call to arms to all young people to register to vote. Let's be clear about the danger: a decision not to vote is a decision to let someone else decide your future.
"Young people can decide this referendum. If they don't use their vote, the danger is this referendum will be lost.
"If young people don't want the Leave campaign to narrow the horizons of the world that they will live in, it is vital that young people register and vote," the former Labour leader was saying.
Leave campaigner and Tory MP James Cleverly hit back, saying: "Ed Miliband's cynical attempt to convince young people otherwise will fool no-one. After his failed efforts to court the youth vote via Russell Brand last year, you'd think he would know better."
In an apparent swipe at pro-Remain Home Secretary Theresa May, Ms Patel wrote in The Sun: "There is another great bonus of leaving the EU, we'll be able to design a new immigration system that brings the chaos under control and helps the economy.
"It's uncontrolled and uncontrollable while we remain in the EU. If the Government seriously believed the doom-laden propaganda they have been pumping out about the horrors of life after the EU, they would never have called this referendum in the first place.
"The Government thinks it can predict what will happen in the economy in 2030. Last November,the Treasury said there was going to be a windfall of £27 billion from unexpectedly high tax receipts.
"When the Budget came round in March, that had turned into a £56 billion shortfall. Fourteen years ahead? They can't even predict 14 weeks ahead."
The move came as Mr Cameron said he would not reconsider comments that Mr Johnson, George Osborne and Mrs May were all figures who could step into his shoes when he steps down, as he has promised he will before the next general election.
The PM said: "I wouldn't withdraw any of the things I've said. The Conservative Party is lucky to have big substantial figures within it and that's certainly the case. But on this one, I think he is on the wrong side."
Meanwhile, prominent Leave campaigner and Tory MP Jacob Rees-Mogg stood by a report by the Treasury select committee he is a member of which strongly criticised statements from both sides in the campaign.
The committee said Vote Leave's "core" campaign claim that Brexit would deliver a £350 million-a-week windfall was "highly misleading".
And it said the Remain side's claim that families would be £4,300 a year worse off if Britain was outside the EU was "likely to be misconstrued by voters" and had "probably confused them".
While the findings are embarrassing for both sides, the committee chairman, Tory MP Andrew Tyrie, said Vote Leave were guilty of "by far the most serious" offence.
He called on them to repaint their battle bus, which has been touring the country with Boris Johnson and other prominent Out campaigners and carries the slogan: "We send the EU £350 million a week - let's fund our NHS instead."
However, Mr Rees-Mogg told The Press Association: "I care nothing about the bus. I am not concerned about charabancs. That is not at the heart of the debate.
"I have always used the net figure. What is far more shocking is that the Chancellor has been using a figure he knew would be misleading."