With Britain's membership in the European Union on the line, campaigners from the prime minister on down blanketed the country Wednesday trying to convert the undecided on the final day before the crucial vote.

Outlining his vision of a future with Britain retaining its position in the 28-nation bloc, Prime Minister David Cameron bristled at the notion that the country would be headed in the wrong direction if the "remain" side prevailed in Thursday's vote.

"We are not shackled to a corpse," Cameron told the BBC. "You can see the European economy's recovery. It's the largest single market in the world."

Pushing for a British exit, or Brexit, the most notable figure on the "leave" side, former London Mayor Boris Johnson, mugged for the cameras at the Billingsgate Fish Market in East London and pretended to kiss a fish - a not-so-subtle reminder that this is an island nation that takes great pride in its independence and self-assurance.

"It's time to break away from the failing and dysfunctional EU system," Johnson said. "It's time to have a totally new relationship with our friends and partners across the Channel."

Wednesday's feverish campaigning took place even as mourners gathered in London and other world capitals to honor the memory of Jo Cox, the youthful pro-EU Labour Party lawmaker who was stabbed and shot to death last week in her Yorkshire constituency.

Speaking to a crowd of 9,000 in Trafalgar Square on what would have been Cox's 42nd birthday, her husband, Brendan, said that Cox "feared the consequences of Europe dividing again" and urged people to follow her example.

The motive for the killing is unclear but the rare slaying of a politician cast a shadow over a divisive campaign that has been unusually heated, even by the lively standards of British politics.

Nigel Farage, the outspoken leader of the U.K. Independence Party, resisted fresh calls to apologize for a controversial poster showing hundreds of non-white migrants making their way across Europe alongside the words, "BREAKING POINT."

The poster, labeled racist by opponents, was unveiled hours before Cox was killed.

"I apologize for the timing and I apologize for the fact that it was able to be used by those who wish us harm," Farage said. "But I can't apologize for the truth."

"This was a photograph that all newspapers carried. It is an example of what is wrong inside the European Union," he said.

High profile political, military and business figures also weighed in on the debate as the final hours of campaigning ebbed away.

NATO Secretary-General Jens Stoltenberg told The Associated Press in Brussels that a British exit from the EU could weaken the trans-Atlantic alliance.

"We are faced with so much uncertainty, so much unpredictability, with terrorist threats, with a more assertive Russia in the east," Stoltenberg said. "I believe that a more fragmented Europe will be something which will only add to the uncertainty which surrounds us."

European Commission President Jean-Claude Juncker flatly rejected suggestions that Britain might be able to negotiate better terms with the EU if it votes to leave. "Out is out," he said.

The reach of the EU into every aspect of life has made the issues at stake far more complex than in a general election. And while the vote is final - unlike in an election where the results can be reversed in the next term - it is not legally binding and Parliament would have to vote to repeal the law that brought Britain into the EU in the first place.

A vote to leave would invoke Article 50 of the Treaty on European Union, which allows a member state to withdraw. That has never been done before and would trigger a period of uncertainty during years of negotiations on the relationship between the EU and the U.K.

Much of the debate has hinged on the economy. From the international banks in the skyscrapers of Canary Wharf to the home of Britain's financial industry in the City of London, business has awaited Thursday's vote with trepidation. Many fear a vote to leave would undermine London's position as one of the world's pre-eminent financial centers and damage an industry that underpins the British economy.

Leaders of about half of Britain's largest companies made a last-ditch appeal to their employees to vote to remain in the EU.

In a letter to the Times on the eve of Thursday's vote, some 1,285 business leaders - including representatives of half of the FTSE 100 businesses - argued that a vote to leave would hurt the British economy.

Similar letters have been released in the course of the acrimonious campaign. But Wednesday's letter was clearly meant to make the 1.75 million people employed by the signatories think twice about their vote.

"Britain leaving the EU would mean uncertainty for our firms, less trade with Europe and fewer jobs," the letter said. "Britain remaining in the EU would mean the opposite: more certainty, more trade and more jobs. EU membership is good for business and good for British jobs. That's why, on June 23, we back Britain remaining in the EU."

Among the companies represented were Barclays, Standard Life and Anglo-American.

Stock markets and the pound continued to rise, indicating that investors think the "remain" side will win. Markets are likely to be jittery, though, as the vote is expected to be tight and a decision to leave would create huge uncertainty. U.S. Federal Reserve chair Janet Yellen warned Tuesday that the upcoming vote "could have significant economic repercussions."

The betting markets stood solidly by the "remain" side, however. The Betfair exchange said "remain" is now at 76 percent probability. In a statement, it said some 80 percent of the 1 million pounds placed during and after a BBC debate on the issue Tuesday were for "remain."