The London assembly member Shaun Bailey will be the Conservative candidate to take on Sadiq Khan for mayor of London in 2020, winning the nomination in the runoff on Friday.

Bailey beat fellow assembly member Andrew Boff and businesswoman Joy Morrissey after a vote by London Tory members. Bailey, a former special adviser to David Cameron, called the nomination “a great honour” and said he would hold Khan to account.

“For someone who grew up in a council house in one of the poorest parts of London, securing this nomination is proof that our city truly is the place where anything is possible,” he said.

“It’s why I’ve spent my entire adult life trying to repay its generosity, either through my 20-plus years of youth work helping to steer children away from a life of crime and towards work opportunities, or my current involvement on the London assembly, where I have done my best to hold the current mayor to account.”

Bailey, 47, said he would focus his campaign on violent crime and the transport budget, as well as housing. “After two and a half years of Sadiq Khan, it is clear we cannot afford more of the same,” he said.

“I look forward to meeting with Londoners in every borough and continuing the conversation about how we can do better to make sure London works for everyone.”

The Conservative party chairman, Brandon Lewis, said Bailey was “passionate about London and has the ideas London needs to continue to be the best capital city in the world”.

He added: “London is being let down by Sadiq Khan, who keeps breaking the promises he made to Londoners on crime, housing and transport. I know Shaun will hold him to account and outline his positive vision for London.”

Bailey was endorsed by the London Evening Standard, which is edited by the Conservative former chancellor George Osborne, and was considered a frontrunner for the nomination.

He was a youth worker who became an adviser to Cameron on young people and crime before being elected to the London assembly. Bailey was selected as a candidate in the 2017 election in the safe Labour seat of Lewisham West, where he lost to Ellie Reeves.

Bailey received 43% of the vote in first round of voting, Boff received 35% and Morrissey won 21%. Bailey defeated Boff in the runoff of second preferences, the party said.

Boff was the only candidate of the three who publicly disavowed the campaign by former candidate Zac Goldsmith, which was criticised for associating Khan with alleged extremists before he became an MP.

The Muslim Council of Britain wrote to all three candidates last month “to ask for your views on Mr Goldsmith’s strategic campaign to focus on Sadiq Khan’s faith and background in an effort to win the election”.

The MCB said it was “deeply disappointing” that Bailey and Morrissey did not publicly distance themselves.

Marsha de Cordova, the Labour MP for Battersea, said Bailey had helped implement £1bn cuts to the Metropolitan police and millions more from London’s youth services during his time in Downing Street.