Liberal Democrat leader Tim Farron has claimed new party member Rachel Johnson was a “fantastic addition to the team” and did not rule out her running for mayor in 2020.

Writing in her column in The Mail on Sunday, Ms Johnson said she would vote Lib Dem as a “protest vote” – and that she was a Lib Dem member “for now” – because the general election had become a “single issue election” on Brexit.

Mr Farron said on The Andrew Marr Show: “Well, Rachel’s a fantastic addition to the team. We picked the right sibling, or rather, she picked us and it’s an absolute blessing to have somebody...”

Mr Marr interrupted and asked whether Ms Johnson, the sister of pro-Brexit campaigner and Foreign Secretary Boris Johnson, would run for London Mayor.

“I think she’d be marvellous at it,” Mr Farron responded.

“But shall I say that’s an election for three years down the road and we’d worry about that after the one we’re currently fighting.”

Ms Johnson appeared herself on the BBC One programme, and confirmed she had joined the Lib Dems too late to run for a seat in the general election.

Asked about whether she might follow her brother's footsteps into City Hall, she said: “I think that was my own paper possibly making mischief to embarrass me but I don’t know and obviously – I’m not ruling anything in or out.”

Boris Johnson's £350m Brexit claims challenged by Susanna Reid on live TV

“We could be going over a cliff in a hard Brexit,” she said on the show. “And there is only one party out of all the parties that is offering voters a second look at the deal, whether it’s bad deal, a good deal, a red, white and blue deal or a Theresa May Brexit.”

She wrote that the Lib Dems “are the only party that represents me, and the 48 per cent, 16 million others, when it comes to this election and the biggest issue of my lifetime”.

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In the column, she claimed that neither of her Conservative brothers had presented one convincing pro-Brexit argument to change her mind and the Tories had instead shown a “faulty” referendum “bill of goods”.

She argued that the UK needed migration for the economy to thrive, as Brexit secretary David Davis pointed out anticipated GDP growth was already slowing and there would be no promised weekly cash injection of £350 million to the NHS.

“They are campaigning against a ‘hard Brexit’ that would take the UK out of the single market and end free movement of people – and for a second referendum on the terms of any Brexit deal reached with the EU,” she wrote.

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“With that offer on the table, I can’t in all conscience support a party (and I write as a former economics hack on the FT and past Tory voter) that’s committed to sacrificing prosperity on the altar of a false and unfeasible pledge to bring immigration down to the tens of thousands, a target that the Office for Budget Responsibility says will cost £6 billion a year.”

She added that her brother, the former mayor of London, asked her if she had lost her “last remaining marble”, but she said that even if she was going against her brothers’ views, she was “entitled” to have an opinion and had always been “consistent on her position”.

Mr Johnson disagreed with Good Morning Britain host Susanna Reid last week that the £350 million figure for the NHS was “disputed”.

Pressed on whether he still stood by the claim, he replied: “Of course I do.”