From the prime minister to sports and pop icons David and Victoria Beckham, supporters of Britain staying in the European Union urged voters Tuesday to think about future generations when they cast ballots in a referendum that has divided the nation.

With the black door of 10 Downing St. offering a statesman's backdrop, Prime Minister David Cameron appealed directly to EU-wary older voters, saying that leaving the bloc would risk the country's economic security — and younger generations would have to live with the consequences.

"Do think about the hopes and dreams of your children and grandchildren," he said. "They can't undo the decision we take," he said. "If we vote out, that's it. It's irreversible."

The plea came as the already heated campaign moved into its tense final days. Opponents said the statement outside the prime minister's residence betrayed Cameron's worry that the referendum he called to settle Britain's place in Europe could end with the country walking away from the bloc.

Steve Hilton, a former Cameron adviser who now backs a "leave" vote, said Cameron had been "wheeled out by rather panicky spin doctors" to appeal for votes.

Cameron seemed to direct his appeal at those who grew up in the after-shadow of Britain's imperial period, in a nation that had stood against Nazi tyranny during World War II. Polls suggest these voters, many of them Conservatives, are more likely than younger Britons to reject the pan-European EU project.

British Prime Minister David Cameron warned on the steps of Downing Street Tuesday that the U.K.'s decision is 'irreversible.' (Dan Kitwood/Getty)

Cameron tried to assure them that Britain remains a great country — even if it is part of a 28-nation bloc — and that being in the EU allows the country to promote the values it holds dear.

"Brits don't quit," he said. "We get involved, we take a lead. We make a difference. We get things done."

Cameron acknowledged that many Britons didn't vote for him and don't like him.

"Not everyone has been happy with what I have done," he said. "But of this I am convinced ... Britain is better off inside the EU than out on our own."

'Stronger In,' says former Spice Girl

The "remain" camp also got a boost from more popular — and populist — figures when soccer star David Beckham and his designer wife Victoria added their voices to the EU cause.

"For our children and their children, we should be facing the problems of the world together and not alone," he said.

David Beckham, shown last year, has thrown his support behind the "remain" side. (Joel Ryan/Canadian Press/Associated Press)

Former Spice Girl Victoria Beckham later registered her view by posting the symbol of the "Stronger In" campaign on her Instagram page.

The "Leave.EU" group had suggested that Victoria Beckham was a supporter of their side based on a 1996 interview with The Spectator in which she was quoted as deriding Brussels bureaucrats and saying that "we must keep our national individuality."

Keeping things positive

Both sides in the referendum were trying to keep things positive, wary that negative campaigning would turn off voters.

Former London Mayor Boris Johnson expressed disapproval of a controversial poster showing a column of non-white migrants massed at a European border alongside the words "BREAKING POINT" — underscoring a deepening rift among campaigners promoting Britain's exit from the bloc.

Former London mayor Boris Johnson has distanced himself from fellow "leave" supporter Nigel Farage, leader of the U.K. Independence Party. (Matt Dunham/Associated Press)

Johnson, one of the most prominent campaigners on the "leave" side, distanced himself from U.K. Independence Party leader Nigel Farage, who has been widely criticized for the poster.

'Xenophobic undertones'

Britain's anti-EU campaigners have long been bitterly divided but the squabbles appear to be intensifying as Thursday's referendum draws near. Johnson's Vote Leave won Electoral Commission approval to be the official standard bearer of the campaign, but Farage has fought on nonetheless.

Johnson told LBC radio the poster had "xenophobic undertones" and insisted the official Vote Leave campaign had nothing to do with it.

"I didn't like it," he said. "It seemed to be saying that these were bad people coming to our country. That seemed to be the message. I felt profoundly unhappy with it."

Stock markets surge

The poster has deepened the division on the "leave" side, coming into public view as it did in the hours before lawmaker Jo Cox was killed in a knife and gun attack outside a library in her Yorkshire constituency last week.

In an extraordinary session in Parliament on Monday, legislators urged an end to angry and divisive politics in honor of their slain colleague.

The British pound and global stock markets have surged as shock at the death of the pro-Europe Cox seemed to sap momentum from campaigners fighting for the United Kingdom to leave the EU. The pound rose to its highest point since early January in trading Tuesday, hitting $1.47.

Billionaire currency trader George Soros added support to the "remain" case on Tuesday, warning that a vote for Britain to leave the EU will trigger a plunge in the pound — without benefits that can come with a devalued currency.