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The Liberal Democrat bidding to oust Zac Goldsmith in next week’s Richmond Park by-election today claimed a victory for her pro-EU party would be a Brexit “game-changer”.

In an interview with the Evening Standard, Sarah Olney said that the shockwaves would reverberate beyond the constituency and “send a really strong message to Theresa May that Brexit and the way she is currently pursuing it is a real vote-loser for the Conservatives”.

Ms Olney pledged that if elected she would vote against the moving of Article 50 which formally triggers Brexit.

We chatted in her family home in North Kingston, a semi whose walls are festooned with pictures of children Isabel, seven, and Rufus, three. One snap shows their mother in costume as Iolanthe, the Fairy Queen (she is a Gilbert & Sullivan fanatic).

Ms Olney, 39, who works as an accountant at the National Physical Laboratory in Teddington, was not even a member of the Lib-Dems until July last year when, after the collapse of the Lib-Tory Coalition, she decided that “sitting on the sofa shouting at the telly wasn’t enough”.

Her husband Ben is a town planner who — sparking controversy, given her opposition to Heathrow expansion — was revealed this week to have been employed on delivering Terminal 5.

Central to Ms Olney’s campaign is a “clear pledge” to halt Brexit. “We don’t believe even a soft Brexit would be right,” she said. But her stance has been confused by the emergence of a blog, in which she wrote: “Britain is leaving the EU — OK, I accept that. I don’t think we should re-run the referendum.”

Asked if she had changed her mind, she insisted: “I wrote that much longer than a month ago. I think people are getting a bit confused.”

When challenged with a printout showing that her words were written on October 14, Ms Olney said: “No, no, no,” but then added: “I don’t know.” She said that Mr Goldsmith, who has accused her of flip-flopping, was making too much of a stray comment on social media.

“Given that when I wrote that I had no idea I was going to be in a by-election or ... that I was going to be under such scrutiny, maybe I would have chosen my words more carefully if I had been an actual by-election candidate at that time ... I was just a normal person; that was just a thought I had.”

Her explanation is sur- prising because the by-election was widely expected and Ms Olney was already her party’s prospective candidate.

All her old blog posts were hastily deleted after the media took interest in them. She said she had been advised to get rid of “a hostage to fortune” but added: “I slightly regret it.”

Would she vote against Article 50 in all circumstances? She replied that although she was seeking “a mandate” to oppose Article 50 being moved, she might vote for it if Theresa May promised a second referendum on the result of its negotiations.

She bridled when asked about an interview in which she reportedly at-tacked Mr Goldsmith as someone with “wealth stashed all over the world” who does not understand what it means “when the price of Marmite goes up”.

“Did I?” she asked, staring accusingly at her Lib-Dem minder. “It may not have been me doing that. I did not say anything about Marmite. I’m not sure whether I actually said any of those things or whether somebody else may have followed up.”

For most by-election candidates, the pressure can be unbearable. But Ms Olney smiled and said: “It is not going to be devastating for me if I lose. But it would be devastating for Zac — it would absolutely trash his reputation.”

She added: “If I don’t win, I’ve got a nice house, a great husband, lovely children and a good job to go back to.”