British business leaders fear that Labour leader Ed Miliband is mounting a better-than-expected challenge to the Conservative Party, a report has suggested.

The heads of the FTSE 100 companies, said the Financial Times, have become frustrated at the tactics and tone of the Conservative election campaign.

A group of 20 business chiefs told the FT that they were particularly critical of "the strident personal attacks on the opposition" - such as calling Miliband a back-stabber - "and the flurry of big-spending promises that jar with the party's prudent fiscal record".

As one company chairman put it to the FT: "The negative campaign has been disastrous."

"The strength of the performance of the coalition in terms of delivering real growth and real jobs has become almost background noise," said another, speaking on condition of anonymity. One FTSE 100 chief said: "Why not play the positive economic note? There is a good story to tell."

The corporate chiefs who spoke to the FT asked for anonymity because they said they did not want to undermine the Tory campaign in public.

One FTSE 100 leader suggested the pledge for a real-terms freeze on rail fares contradicted the Tories' promises of responsible fiscal management. "If you intervene in rail prices, you legitimise Ed [Miliband] on intervening in energy prices," he said.

Several others suggested the Tories had underestimated Miliband's abilities. "Every time Miliband is visible, he appears credible," one business leader told the FT.

Mick Davis supports the Tories\' programme

The Tories, however, did have some support in the City. The FT reported that Mick Davis, former chief executive of Xstrata who now runs a private equity group, said David Cameron was "right to highlight the real risk an SNP-Labour government poses to investment, to jobs and to the economy. The best outcome for business is a strong, majority Conservative government that sticks to its economic programme."

And Ian Taylor, chief executive of oil trader Vitol, also backed the campaign, telling the paper: "The success of UK's economic recovery demonstrates David Cameron has the right plan for the future. British jobs and quality public services depend upon strong economy and stable pro-business environment."

Labour's plans would add £90bn to the national debt, claims IFS

Meanwhile, figures published by the Institute for Fiscal Studies have revealed that a ­Labour government would increase tax revenues by £12bn more than the Tories by the end of the decade.

A report in The Times said it had also estimated that ­Labour's slower approach to clearing the deficit would add £90bn to the national debt, leaving Britain less able to withstand another global downturn.

The think tank's analysis showed that the Tories, by contrast, were offering voters £7bn of tax cuts by the end of the next parliament. However, the IFS did query the Tories' claim to be able to fund the giveaways, saying they relied on "highly uncertain" revenues from tax avoidance.

The think-tank also criticised the Tories for failing to spell out how they would achieve £30bn of welfare and spending cuts deeper than those already imposed since this was "not mentioned in their manifesto", according to Paul Johnson, the director of the IFS.