Former government minister and United Nations official Lord Malloch-Brown chairs the Best for Britain campaign part-funded by billionaire financier George Soros.
The peer said he was “profoundly” motivated by the feelings of his children, their friends and the youth groups his campaign was working with.
He said: ”They are all really angry with older people.
“They feel utterly betrayed – and they feel that the actuarial rules of life are moving in their favour and that within a matter of four, five 10 years, they are going to be an overwhelming majority in this country.”
He agreed with a Telegraph podcast interviewer that this would happen “as all the Brexiteers die off … because under 55 there was a huge majority in this country [for Remain], tailing off as you got to 55”.
His campaign wants MPs to vote against the terms of Theresa Mays potential Brexit deal with Brussels
Ideally Lord Malloch-Brown said he would like Parliament given the option of backing staying in the EU if the deal would clearly make Britain worse off.
But it would also be “a perfectly reasonable outcome” if Mrs May’s deal was defeated, leading to a new Tory leader who put the package to a second referendum.
He said most MPs were “remainers”, according to their referendum votes.
“I agree about respect for the will of the people, but democracy is several things and one is the right to hear new facts, the right to change your mind, to have a fresh vote,” he said.
He estimated his campaign’s current chances of winning its “uphill fight” at 40 per cent: “But is it worth fighting? As a democracy and a patriot I believe it profoundly is.”
Matthew Elliott who led the successful Vote Leave referendum campaign, retorted: “It’s a complete myth that it’s just old people who voted for Brexit.
“It’s like when people say ‘everybody in London voted remain’ whereas more people voted to leave in London than voted for Labour mayor Sadiq Khan.”
Mr Elliott added that “if the Leavers who now support Remain exist, where are they in this debate?”
He also thought Lord Malloch-Brown was being “wildly optimistic” to suggest that the Remain camp’s chances were as high as 40 per cent.
Meanwhile it emerged that anti-Brexit activists plan to launch a concerted new wave of action within weeks to try to change Brexiteers’ minds and preserve the closest possible links with the EU.
Best for Britain is considering a summer rock festival in London as well as planning a mobile phone app and billboard campaigns to press MPs to vote against the Government’s Brexit deal or at least to keep Britain in the EU customs union.
It is drawing inspiration from Jeremy Corbyn’s backers in the hardleft group Momentum which used social media effectively in last year’s general election to motivate and direct Labour activists to boost the party’s vote in key seats.
Previous research has shown that a majority of 18 to 34 years olds across all social groups voted in the 2016 referendum to remain in the European Union and a majority of those aged 55 and over voted to leave.
But economic backgrounds were a factor with working class 35 to 54-year-olds more likely to vote to leave and their middle class contemporaries to back staying.
Tonight Best for Britain announced a new partnership in which it will donated £30,000 to the anti-Brexit Our Future, Our Choice campaign for young people.
Best for Britain says this week’s controversy over Mr Soros’s backing has triggered ordinary people to donate tens of thousands more to its work.