By Jenny Gross and Nicholas Winning 

LONDON--Business leaders from more than 100 companies released a letter Wednesday backing the U.K.'s Conservative Party, marking a significant escalation in the intervention from British business ahead of a national election.

The letter, signed by senior representatives of corporate giants such as BP PLC and Prudential PLC as well as from many smaller companies, voiced support for Prime Minister David Cameron's Conservatives' policy of lowering the corporate tax rate to 20%. They also warned that a change in course for Britain's economy would "put the recovery at risk," according to the letter, published in the U.K.'s Telegraph newspaper.

Labour countered that corporate taxes would remain competitive if it was in government and has said that a vote for the Conservatives--who have pledged a referendum on whether the U.K. should leave the EU--could augur the U.K.'s departure from the world's largest trading bloc.

At stake when Britain goes to the polls May 7 are issues with far-reaching implications, including the U.K.'s membership of the European Union and management of one of the world's largest economies.

But some in business say this election presents them with a difficult choice.

Support for the center-right Conservatives could usher Britain out of the European Union because Mr. Cameron has pledged--if he wins the election-- to hold a referendum on whether the U.K. should exit. But backing center-left Labour could lead to a series of market interventions, as its leader has promised to introduce measures such as freezing retail energy prices and forcing banks to sell off branches to encourage more competition.

"Labour would be tough on business--and might be perceived as lacking fiscal responsibility. A Conservatives-dominated cabinet would pave the way for an unsettling referendum in 2017 on the U.K.'s European Union membership," investment fund BlackRock Inc. said in a paper issued this week on the election.

The release of Wednesday's letter comes two days after Labour took out a full-page advertisement in another U.K. newspaper, the Financial Times, warning "the biggest risk to British business is the threat of an EU exit." The ad included quotes from a number of business people--including those from Siemens U.K., BHP Billiton and Nomura--echoing that sentiment.

The move backfired slightly after Siemens said the quote from the head of its U.K. unit hadn't been intended for use in a political campaign. Siemens said the comment by the executive was a matter of public record and that "we are very supportive of Britain remaining in a reformed EU" but that it wasn't made aware the quote would be used for a "Labour Party campaign."

Aides of Labour leader Ed Miliband defended their use of the comments, saying they were public statements made in the past by the business leaders and nobody was challenging their accuracy.

A key battleground in the close-fought election is who can best manage the U.K.'s economy, which is growing again after several years of little or no growth in the wake of the recession. But Britain continues to struggle with a large budget deficit and weak productivity.

Data released this week showed the British economy grew at a faster pace in 2014 than previously thought, at a revised 2.8%. But the amount of output produced by British employees for every hour of work fell 0.2% in the final quarter of last year. This means labor productivity remains below where it was in 2007, before the economic downturn.

Labour has struggled to convince voters it will put aside its bigger-spending practices of the past and balance the budget while delivering jobs. The Conservatives also accuse Labour of being antibusiness for supporting measures such as increasing the top income-tax rate to 50%.

Nick Robertson, CEO and founder of online fashion retailer Asos PLC, said Wednesday on a conference call that he supports the Conservative-led government's reduction of corporate taxes. He said it encourages companies like Asos, which has the "lion's share" of its sales outside the U.K., "to remain in the U.K. despite our global footprint."

Labour leader Mr. Miliband rejects the antibusiness label and argues that Mr. Cameron's promise to hold a referendum on whether the U.K. should leave the EU is destabilizing for industry and is the "biggest risk to British investment and jobs in this decade."

Businesses in Britain often complain about regulations that emanate from Brussels, but many say they would fear losing unfettered access to the world's biggest economic grouping if it left the EU. Virgin boss Richard Branson has warned that leaving the EU would do enormous damage to entrepreneurs throughout the U.K.

Martin Sorrell, chief executive of U.K. advertising group WPP PLC, said this election presents business with a "difficult choice." In an interview with British Broadcasting Corporation on Wednesday, he said the uncertainty an EU referendum would generate would discourage businesses from taking risks and investing. And, if the U.K. did vote to leave the EU, it would have "negative effects on the growth of the economy," he said.

But not all business leaders advocate remaining in the EU.

Simon Wolfson, chief executive of U.K. retailer Next PLC, said he favors leaving the EU because it would liberate the U.K. from debilitating EU regulation and increase trade opportunities with the rest of the world. In a speech last month, however, he noted that leaving the bloc also would come with big risks. People could interpret the U.K. leaving the EU as the country taking an opportunity to build national walls rather than tear them down, he said.

--Simon Zekaria, Jana Simmons and Christopher Alessi contributed to this article.

Write to Jenny Gross at jenny.gross@wsj.com and Nicholas Winning at nick.winning@wsj.com

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