Media playback is unsupported on your device Media caption George Osborne claims that a vote for Labour means the economic recovery will be "put at risk"

The Conservatives have welcomed what they say is an "unprecedented" pre-election endorsement by business chiefs but Labour has dismissed the move.

More than 100 bosses signed a newspaper letter backing cuts in corporation tax and other policies since 2010, saying they had been "good for business".

Labour said the letter was "organised" by the Tories and it was "no surprise" that bosses wanted lower taxes.

It came as Labour promised new rights for workers on "zero-hours" contracts.

Ed Miliband said a future Labour government would guarantee zero-hours workers the right to a formal contract after 12 weeks of regular work, a move which he said would reduce economic insecurity but which was criticised by employers.

In other developments in the election campaign:

Deputy Prime Minister Nick Clegg says he will win his Sheffield Hallam seat after a poll by Lord Ashcroft suggested he was trailing Labour

The Lib Dems have also promoted their plans to triple paternity leave entitlement

Shadow Chancellor Ed Balls denies he intends to lower the point at which taxpayers pay the 40p tax rate

In Scotland, Labour says it is within "striking distance" of the SNP and will use its first Budget to ensure £800m more spending

UKIP says 16 and 17-year olds and EU migrants should not be able to vote in any future EU referendum

On a visit to a school in Kent, Samantha Cameron says her husband "doesn't seem too nervous" ahead of Thursday's leaders debate

Boris Johnson is to launch the Conservative campaign in London

Welsh politicians clash over the economy in an election debate

In their letter to the Daily Telegraph, the businessmen - including BP chief executive Bob Dudley, Prudential boss Tidjane Thiam and Nick Robertson, chief executive of ASOS - back the reduction in corporation tax from 21% to 20% which took effect on Wednesday.

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Pollwatch

David Cowling, editor, BBC Political Research

YouGov's first poll following their Sunday 4% Labour lead had Conservative and Labour level-pegging on 35%; and TNS had the Conservatives one point ahead of Labour (33% v 32%) with UKIP on 16% - their highest figure in a campaign poll so far.

A ComRes poll of 40 Labour seats in Scotland confirmed other national polls, with a 19% swing from Labour to the SNP.

There was better news for Labour in two London-wide polls. ComRes had Labour on 46% and YouGov on 45%, with the Conservatives hovering around their 2010 share of 34%.

ComRes represented a 5.5% swing to Labour and YouGov a swing of 4%.

The Lib Dems were down from 22% in 2010 to 8% now.

UKIP continued to underperform in London with around 8-9%; and the Greens will be disappointed that they were on 4% in both polls in a city where they have performed better than average in the past.

BBC poll tracker

The business leaders, who signed the letter in a personal capacity, say that the present government has supported investment and job creation.

"We believe a change in course will threaten jobs and deter investment. This would send a negative message about Britain and put the recovery at risk," they said in the letter.

About 20 of the signatories are thought to be long-standing Conservative supporters.

Media playback is unsupported on your device Media caption Ed Miliband said he was not surprised business leaders were in favour of lower tax

But the paper said five signatories had previously supported New Labour: the entrepreneur Duncan Bannatyne, the hotelier Surinder Arora, chairman of Dixons Carphone and Talk Talk Sir Charles Dunstone, theatrical producer Sir Cameron Mackintosh and businessman Moni Varma.

The headline rate of corporation tax - paid on company profits - has fallen steadily under the coalition government, down from 28% when it came to power in 2010.

The Conservatives say the UK has the most competitive corporate tax regime in the G7 but Labour says small firms have not benefited in the same way as multinationals.

Expert views: By Kamal Ahmed and Robert Peston

Whatever the "anti-business" feeling among some of the public - linked, of course, to the financial crisis - politicians believe that the backing of chief executives is a positive.

Read more from business editor Kamal Ahmed

Neither business leaders nor economists have a monopoly of wisdom on what's good for Britain or are free from political bias.

Read more from economics editor Robert Peston

Labour has released its own open letter from "working people" supporting its approach.

The letter, signed by more than 100 people "from all walks of life", says the country needs a Labour government "to put working people first".

Shadow business secretary Chuka Umunna said Labour would maintain a competitive tax regime while prioritising support for small firms by cutting business rates on 1.5 million small business premises.

"This was a letter organised by the Conservative Party in a Conservative-supporting newspaper," he told the BBC.

"We've got almost five million businesses in our country. At best you could say the people who signed this represent 0.002% of them."

Image copyright Reuters Image caption Samantha Cameron has made her first solo campaign appearance

Lib Dem Business Secretary Vince Cable told BBC Radio 4's World at One the letter appeared to have been planted by the Conservatives - and he warned the signatories to "be careful what they wish for".

He said the list was "not a representative sample" of business people across the UK, but declined to name businesses that supported the Liberal Democrats, saying "it's not relevant".

However, Chancellor George Osborne said those behind the letter were a "roll call of British economic success, innovation and job creation".

"An intervention on this scale and with this clarity from Britain's business leaders is unprecedented in any recent general election," he said.

During the 2010 election, a group of business leaders sent a letter to the Daily Telegraph backing the then Conservative opposition's plans to stop rises in national insurance contributions. In 2005, bosses backed Labour in letter to the Financial Times while Labour also received business endorsements in 2001 and 1997.

Speaking in Yorkshire, Mr Miliband said the letter reflected the Conservatives' view that wealth should "trickle down" from a "few companies and individuals at the top".

He also pledged more security for workers on zero-hours contracts, where employees are only paid for the hours the employer needs them and are not guaranteed any work, saying their use had increased by 20% in the past year.

If elected, he said Labour would change the law to give employees the legal right to request a formal contract after three months of continuous work rather than after a year.

Analysis by political correspondent Iain Watson

Labour strategists tell me they didn't know the pro-Conservative business letter was going to appear in today's Telegraph but they insist they are "not unhappy" it coincided with their own policy announcement on zero-hours contracts.

Partly because the sharp dividing line between boardroom and shop floor, they believe, will help motivate their own voters to make the journey to the ballot box.

But they insist that their announcement will appeal beyond their own core vote.

Their argument is that insecure low-paid employment leads to low productivity and is bad for the economy as a whole. Nonetheless, in 2005, more than 60 business leaders wrote to the FT backing Tony Blair.

So while Ed Miliband insists he is in the right place on standing up for working people, his 'one nation' Labour Party hasn't achieved his predecessors' feat of gaining endorsements from unions and big companies.

Indeed it feels we are witnessing the sharpest political divide for more than two decades.

"We have an epidemic of zero hours contracts in this country," he said.

"You should not be left at the beck and call of an employer who can ask the world of you but give you no security in return," he said.

"It is not fair, it is not good for business and we have to put a stop to it."

The proposal, which has been welcomed by the unions, would see an end to more than 90% of existing zero-hours contracts, Labour said.

But Conservative sources said just 2% of workers were on such contracts and nearly 40% of them already worked full time while the Lib Dems said the proposals went "too far".

Mr Osborne said he would find it "very difficult" to live on a zero-hours contract. "Some zero hours contracts people want," he said.

"But for people who want to work longer hours, the answer is to create more jobs."

Mr Cable said Labour had "wildly exaggerated" the scale of the contracts.

The CBI business group warned against "playing with the jobs that many firms and many workers value" while the Institute of Directors called it an "example of politics trumping good policy".

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