Failing to vote in the EU referendum risks creating a "lost generation" of young people, Paddy Ashdown warned ahead of a campaign blitz urging people to register for the poll.
The Liberal Democrat former leader accused Brexit campaigners of having a vision that would leave Britain "divided and weakened", and said the risks of not having a say over the country's future in the 28-member bloc were "very clear".
Around six million residents have yet to fill out the forms that will allow them to take part in the June 23 vote - and more than one million of those are under-25.
Lord Ashdown is joining forces with high profile figures from across the political spectrum in a push across the UK to boost turnout.
"The risks of not registering, and not voting are now very clear," he said. "The Leave campaigns' 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."
Britain Stronger In Europe is staging 30 rallies across the UK in the major voter registration drive.
Labour former leader Neil Kinnock, Green MP Caroline Lucas, Labour's Harriet Harman and Vernon Coaker and Conservative minister Anna Soubry are among those urging the public to register by the June 7 deadline.
Lord Kinnock said: "Time is running out for people to have their say in this referendum and that's why we are mobilising grassroots volunteers and leading public figures in every region and nation on Saturday to drive up registrations and encourage postal votes.
"It is crucial that as many people as possible have their say on the future of this country, rather than leaving it to others to make the choice for them.
"That's especially vital for young voters who will live longest with the Referendum outcome.
"We will be reminding people of that this Saturday and demonstrating to them why Britain is stronger, safer and better off in the EU."