LONDON (AP) -- Campaigners on both sides of the crucial vote on whether or not Britain should remain in the European Union crisscrossed the country Wednesday, their last day to win support from the undecided.

Prime Minister David Cameron outlined his vision for a future with Britain retaining its place in the 28-nation bloc, bristling at the notion that the country would be headed in the wrong direction if it stayed in. He flatly rejected the charge that the institution is moribund.

"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."

The most notable figure in the "leave" campaign, former London Mayor Boris Johnson, kicked off a whirlwind tour of England as he pushed for a British exit -- or Brexit. Touring the Billingsgate Fish Market, Johnson mugged for the cameras with fish in hand -- a not-so-subtle reminder that this is an island nation -- and one very proud of its independence and self-assurance.

"It's time to have a totally new relationship with our friends and partners across the Channel," Johnson said. "It's time to speak up for democracy, and hundreds of millions of people around Europe agree with us. It's time to break away from the failing and dysfunctional EU system."

Britain goes to the polls Thursday after a campaign that has been both heated and complicated. The reach of the EU into every aspect of life has meant that all sorts of groups -- from scientists to CEOs -- have registered opinions on whether to stay or go.

The stakes are high as the vote is final -- unlike an election in which the results can be reversed in the next term. However, the vote 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. The article has never been invoked and it 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 traditional home of Britain's financial industry in the City of London, business has largely awaited the referendum with trepidation and caution. Many fear a vote to leave would undermine London's position as the world's pre-eminent financial center 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 for remaining in the European Union.

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

Similar letters have been released in the course of the acrimonious campaign. But Wednesday's letter is 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."

The companies represented include 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, however, as the vote is expected to be tight and a vote 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 head of the European Central Bank, Mario Draghi, also said the consequences of a vote to leave would be difficult to assess.

"This lack of precision from two of the most eminent central bankers in the world with respect to the global consequences of a vote to exit was a refreshing outbreak of honesty in contrast to the very precise warnings that the U.K. public has been bombarded with," said Michael Hewson, the chief market analyst of CMC markets, citing warnings from the International Monetary Fund, the Bank of England and the Organization for Economic Cooperation and Development -- all of which signaled dire consequences if the country were to exit.

The betting markets solidly stood by the "remain" side. The Betfair exchange said remain is now at 76 percent probability. Some 80 percent of the 1 million pounds placed during and after a BBC debate on Tuesday was on "remain," the exchange said in a statement.