"The risks of not registering and not voting are now very clear," declared Lord Ashdown. "The Leave campaign’s vision of Britain is one that is divided and weakened. We cannot let them succeed.
"If we do not now pull out every stop to get people registered and get them to vote, then we risk a lost generation for Britain's young people outside the EU," he added.
Britain Stronger In Europe is staging 30 rallies across the UK in the major voter registration drive with political figures such as Labour’s Neil Kinnock and Harriet Harman, the Greens’ Caroline Lucas and Tory minister Anna Soubry all taking part to urge people to register by the June 7 deadline.
Meantime, Gordon Brown today joins other former Labour leaders to persuade the party's working class supporters to turn out and vote Remain.
In a joint letter, they warn that Labour communities have the “most to lose if we leave but also the most to gain if we remain".
In a speech at the Hay-on-Wye book festival, the former Prime Minister will say the Remain camp is relying on non-Conservative supporters - nine million who voted Labour in 2015 and five million backers of other "progressive" parties - for the bulk of the votes it needs to keep the UK in the EU.
But he will warn polling suggests a large number of Labour supporters could stay at home while many skilled workers who make up much of the party's core support are thought to be considering voting Leave.
The Scot will call for a "positive, principled and progressive" message to persuade these people to back Remain to support jobs, security and fairness.
The move comes after David Miliband, the former Foreign Secretary, said Labour needed to work harder to make its own "distinctive" case for Britain staying in the EU.
Yesterday, Nigel Farage argued the referendum campaign’s new focus on immigration represented a "turning point" in favour of Brexit, saying he now confidently predicted victory for Leave.
The Ukip leader claimed the "conversion" by senior Vote Leave Tories to his party’s long-standing call for a points-based system on immigration together with David Cameron's rough ride at the hands of TV viewers on Thursday night showed the tide had turned.
He accused the Prime Minister of showing a "sheer level of dishonesty and deception" in his response to voters' questions about controlling Britain’s borders and declared: “All I have to do next week is tell the truth because the British public are ready to hear it." Mr Farage will appear in a live set-piece debate on an ITV programme with Mr Cameron albeit not at the same time.
In a separate development, the Ukip leader announced he would be leading a 60-boat flotilla up the River Thames to Westminster to call for Brexit and to highlight the plight of British fishermen, claiming virtually the entire industry backed Brexit because of EU fishing quotas.