Media playback is unsupported on your device Media caption David Cameron: "We've got a fight on our hands, and we're going to win"

David Cameron has promised to work "the hardest I've ever worked in my life" to secure a Conservative victory on 7 May.

He made the pledge after 5,000 small businesses signed a letter calling for the Conservatives to be "given the chance to finish what they started".

Printed in the Daily Telegraph, it says the coalition has kept the UK "open for business" by keeping interest rates and inflation low and tackling the deficit.

Labour says the Tories "let down" small firms by failing to boost finance.

Business spokesman Chuka Umunna said the government had overseen an increase in business rates of around £1,500 and Labour would cut - then freeze - the rates.

But speaking to an audience of small business leaders in north London, David Cameron said the small business community was the "magic ingredient" and "the backbone of our economy".

'10-day fight'

He said he wanted success on 7 May "for the small business that works round the clock to build something, employ people and make a success... for those who go out and work hard to get that security for their family, to make the most of their lives."

He added: "If you think I'm going to roll over in the next 10 days and let Ed Miliband and Alex Salmond wreck that, you've got another thing coming.

"We've got a fight on our hands and we're going to win that fight."

Mr Cameron reiterated promises to keep taxes low, cut red tape, treble the number of start-up loans and invest in infrastructure like superfast broadband, arguing there were too many "non-spots" where people had no access at all.

He spoke out after 5,000 small and medium-sized firms signed up to a letter on the Conservatives' website supporting the party's business policies. The 5,000 signed up employ a total of about 100,000 people. There are, at least, 5.2 million such businesses in the UK.

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The text praises the coalition's efforts to "get the economy moving again by tackling the deficit, helping to keep interest rates low and inflation down".

It states: "We've been helped by their steps to lower taxes, reduce red tape, simplify employment law and get the banks lending."

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After repeating the Conservatives' claim that small businesses have been creating 1,000 jobs a day, the letter adds: "We would like to see David Cameron and George Osborne given the chance to finish what they have started."

However, one of the 5,000 businesses apparently signed up to the letter has withdrawn its support, claiming it never put its name to the document in the first place.

Aurum Solutions has been removed from the letter to the Daily Telegraph after insisting its sales director did not sign it and that the company's policy was "politically unaligned".

And it has emerged that two of the signatories, Chris Pearson and Nicola Wilson, are Conservative election candidates while a third is a councillor.

But businesswoman and Tory peer Karren Brady, who is the party's small business adviser, denied the letter was a stunt, telling the BBC that "every single person" who signed it had been "verified".

The letter's publication coincides with the launch of the Conservatives' small business manifesto, which aims to see 600,000 new firms created each year by 2020.

Media playback is unsupported on your device Media caption The BBC's Christian Fraser runs through the best tweets

The small business manifesto also includes a commitment to review the "disadvantages faced by the self-employed" - now 15% of the labour force - including accessing maternity pay and building up pensions.

Mr Cameron said the Conservatives were the party of "the small businesses, the entrepreneurs, the techies, the roof tilers, the retailers, the plumbers, the builders" and said a Conservative government would continue delivering the "pro-business environment" which small companies wanted.

'Stability and security'

The election, he claimed, was a "pretty binary choice" between the Conservatives and Labour, and he defended his attacks on the risks posed by the opposition and Ed Miliband, whom he described as the "other guy".

"Some people think stability and security is boring, I don't," he said, arguing that certainty helped families to plan and businesses to invest.

And he urged voters to focus on the critical choice facing them before polling day on 7 May, saying he wanted to win for the sake of the country, not himself: "I don't want anyone to think they can opt out of this election.

"We have got 10 days to deliver this and they are going to be 10 days when I am going to work the hardest I ever have worked in my life because there is so much on the line."

The CBI said it supported any efforts to reduce red tape and increase funding for research and development.

However, Mr Umunna said: "The Tories have spent five years letting down Britain's small businesses.

"Government scheme after government scheme designed to boost finance for small firms has failed, and small business lending has fallen by £500m in the last three months.

"With Labour, the tax burden on small firms will be lower than under the Tories."

He said his party would address late payments and "unfair treatment of small suppliers" and set up a British Investment Bank to back small businesses.