LONDON (Reuters) - British Prime Minister Gordon Brown said yesterday the Conservative Party's tax plans did not add up, seeking to regain the initiative on the top election issue after business leaders backed the opposition.

Brown has faced criticism from business groups and more than 60 large employers over his ruling Labour Party's proposal to raise the National Insurance payroll tax. Critics say it would cost jobs and damage recovery from the recession.

Business chiefs reacted with anger after Brown said they had been deceived by the Conservative Party, which supports a more limited tax rise which would effectively exempt seven out of 10 workers.

Brown is aiming to turn the row to his advantage, saying the Conservative plans would require an additional £6 billion of savings this year on top of £15 billion the government has already earmarked.

"Do the British people really want to gamble (their) economic future on the basis of a back of an envelope set of calculations like this?" he told reporters, saying opposition leader David Cameron's "flimsy" plans were not credible.

The row has taken centre-stage since Brown announced on Tuesday that the election, which promises to be the closest race in almost 20 years, would be held on May 6.

Labour is trailing the Conservatives in opinion polls, but the gap has narrowed and the latest surveys point to an inconclusive outcome where no single party would have an overall parliamentary majority.

The latest Reuters/Ipsos MORI poll on Thursday said voters in marginal constituencies narrowly held by Labour which will be crucial in deciding the outcome, remained unconvinced by Cameron and rated Brown more highly.

A strong rebound in Britain's industrial output and house price data on Thursday strengthened economists' hopes that economic recovery remained on track during the first three months of 2010.

The Conservatives are seeking to use business support over National Insurance as evidence they have the right economic plans to help Britain address a record budget deficit and secure recovery from the deepest recession since World War Two.

Cameron said even more company bosses had added their names to the list of backers and the government was looking "rattled".

"You've got some of Britain's biggest and most successful business leaders saying ... the threat to the recovery is not cost savings and cutting out waste, the threat to the recovery is Labour's job tax," he told reporters.

"New Labour was the party supposedly of business and they've now completely fallen out with the businesses that were going to help us to get out of recession."

That view was echoed by Stuart Rose, executive chairman of retailer Marks & Spencer and one of those to have backed the Conservatives.

"This is an important argument and I think to insult the collective intelligence of 60 plus chief executives is not helpful," he told BBC radio.

Labour made a deliberate point of courting business leaders before winning power in 1997. Their continued backing has given substance to the centre-left party's economic credentials, helping it stay in government for 13 years.

Brown insisted Labour was not in a fight with businesses and pointed to the backing of Gerry Grimstone, chairman of insurer Standard Life and a Treasury adviser, who said the Conservative plans were "incoherent".

"It is just not credible to think that our savings can be almost doubled," he wrote in the Financial Times newspaper.