The Conservatives have selected Susan Hall as their candidate for next year’s London mayoral election, with the strongly pro-Boris Johnson, Donald Trump-supporting assembly member likely to be seen as an outsider to defeat Sadiq Khan.

Hall, a member of the London assembly since 2017, and formerly leader of its Conservative group, beat Mozammel Hossain, a barrister with virtually no frontline political experience, in a ballot of Tory members, winning 57% of the vote.

The ballot was originally going to be based on a three-person shortlist but the other candidate, the tech entrepreneur and former No 10 adviser Daniel Korski, dropped out after allegations of inappropriate behaviour towards a woman, which he denied.

Labour immediately sought to portray Hall as “a hard-right politician who couldn’t be more out of touch with our city and its values”, pointing to her vehement support for Johnson, Liz Truss, and even Trump.

An often outspoken character with a robust and active Twitter presence, Hall was criticised by the Conservatives in 2021 for arguing that the deadly storming of the US Capitol by supporters of Trump was the equivalent of UK politicians who opposed Brexit.

She vocally supported Trump before his bid for re-election in 2020, and was seen as the choice for the mayoral race by Johnson, for whom she was a loyal backer.

Khan and his team are likely to also point to Hall’s vehement support for Truss and her short-lived government, including a tweet after the mini-budget that precipitated a fiscal crisis, saying: “Oh deep joy, a proper #Conservative government”.

Her views on immigration may also be problematic in a city known for being more liberal than the UK average, especially on cultural issues, with Hall backing the home secretary, Suella Braverman, over her description of small boat crossings as “an invasion”.

Speaking after the announcement, Hall said she was “a low-tax Tory”, which was why she had supported Truss, but that she accepted “things did not go as they should have done”.

Rishi Sunak’s press secretary distanced the prime minister from Hall’s views about the Capitol violence, but said she had his full support. The press secretary said Sunak had not met Hall recently, but that “I believe he has” met her in the past.

In an immediate hiccup to her campaign, the Conservatives complained to London’s Evening Standard newspaper for putting on its front page a photograph of Hall hoisting her arms in triumph after her win.

The photo was “a clear mockery” and “contemptible – especially as [Hall is] the first female candidate for London mayor from either of the two main parties”, Nickie Aiken, a Tory MP and party vice-chair, wrote in a letter to the paper’s editor, Dylan Jones, which she shared on Twitter.

Hall had been “heavily encouraged to pose with her arms up in the air” by the paper’s photographer, Aiken said, and did so despite initial reluctance.

The Greens have chosen the Hackney councillor Zoë Garbett as their candidate for the election, which takes place in May next year. The Liberal Democrats will choose between a shortlist of the tech industry adviser Rob Blackie and Chris French, a community advocate.

While Khan will be viewed as the favourite, Labour will face the obstacles of incumbency in seeking a third term, controversy over his expansion of the ultra-low emission zone to outer boroughs, and a change to the electoral system, which means votes from lower-ranking candidates can no longer be transferred to the top two.

Hossain, who grew up amid poverty in Bangladesh and moved to London when he was 21, was seen as Downing Street’s favoured choice. However, as a barrister with virtually no political experience before the mayoral race, he was hugely inexperienced – and was labelled “Mysterious Moz”.

The emergence of Hall and Hossain as the only two candidates put to members came after a controversial selection battle in which Paul Scully, the minister for London and the most high-profile candidate vying for the role, was excluded from the shortlist.

Another potential frontrunner, Samuel Kasumu, a former adviser on race issues to Johnson’s Downing Street, had failed to make the initial longlist of eight.

Korski dropped out of the race after Daisy Goodwin, a novelist and TV producer, alleged that he had put his hand on her breast during a meeting at Downing Street when he was an adviser to David Cameron 10 years ago.

Korski said he “categorically” denied the claim, but that it was becoming a distraction to the selection race.