Boris Johnson will take on Ken Livingstone in next year’s London mayoral contest after triumphing in an open primary election in which only just over 20,000 voters took part.

Mr Johnson, the MP for Henley-on Thames, secured 15,661 votes to become the Tory candidate. His closest rival, Victoria Borwick, won 1,869 votes, followed by Andrew Boff with 1,674 and Warwick Lightfoot with 609.

In theory, more than five million London voters supporting any party could have taken part in the contest. People had to register on a premium rate phone line to participate, then vote by post.

No one expected a big turnout because Mr Johnson was the overwhelming favourite. But the number of people voting was lower than in 1999 when Lord Archer beat Steven Norris by 15,716 votes to 6,350 to become the Tory candidate. And that vote involved only Conservative supporters.

David Cameron said: “Boris will be an excellent candidate and I know the party will unite behind him to remove Ken Livingstone from office. It’s about time that all Londoners had the chance to vote for real change.”

Mr Johnson thanked Londoners for the chance to stand against Mr Livingstone. He said: “As I visited all of London’s 32 boroughs in the last few weeks, the message is loud and clear — King Newt’s days are numbered.

“Across London I’ve met people fed up with paying so much to City Hall and getting so little in return.

“The job of the mayor is simple — to get people to work on time, to ensure that people feel safe on the streets, to help people find a place to call home, to celebrate our diversity and to champion our success.

“My determination to lead this city is stronger than ever. After seeing both the good and bad that London has to offer, I am committed to making London greater and standing up for every Londoner that invests so heavily in our city.”

Caroline Spelman, the Conservative Party chairman, said that the primary was “a first for British politics”. “By opening up the process and running open hustings, we have given all Londoners the chance to vote on the Conservative appointment and influence the outcome,” she said.

“The Conservative Party can now unite behind Boris to offer the people of London a real alternative to the misgovernment of London that we have seen for the past eight years.”

Mr Livingstone said: “I will run on my record of building London’s public services — one and a half million more bus trips every day, a beat police team in every neighbourhood, record transport investment, winning the 2012 Olympics and London playing a leading role in tackling climate change, with a clear promise to Londoners that we will build further on these over four more years.

“Boris Johnson proposes to add a lack of managerial competence to the Thatcherite decline and division always represented by London’s Tories.”

Steve Hart, the London regional secretary of Unite, said: “Gaffe-prone Boris Johnson, with his open mouth, disengaged brain record, will spend more time apologising to Londoners than running the capital.

“Londoners should continue to support Ken Livingstone, who is delivering on transport, is the best bet for jobs and economic growth and is a Londoner through and through, not a privileged son of the shires.”

Mr Johnson remains an MP. But he will face a dilemma over whether to carry on at Henley if Gordon Brown calls an early election.

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