THERESA May’s already slim majority in the House of Commons has become yet slimmer, after a shock Liberal Democrat victory in the Richmond by-election.

Tim Farron said his party’s victory was a decisive rejection of a hard Brexit. The European Parliament’s lead Brexit negotiator Guy Verhofstadt even tweeted his congratulations to winning LibDem candidate Sarah Olney, saying “Europe is watching and we are proud”.

But, as with all by-elections, it had its own particular set of circumstances. That vote was caused after Tory MP Zac Goldsmith resigned in protest at the Government’s decision to approve a third runway at Heathrow Airport, with thousands in his constituency under the flight path.

The Tories decided not to stand a candidate in the by-election, and senior Conservative MPs even campaigned for Goldsmith. Though he was technically an independent, he was clearly a Tory in all but name.

But yesterday, after a month of campaigning that saw the LibDems putting every activist and door knocker into the constituency, they overturned the Tories 23,000 majority, with Olney winning by 1,872 votes. The party reckoned they’d spoken to 52 per cent of constituents and claimed they knocked on 20,000 doors last weekend.

At last year’s General Election, the Liberal Democrats were routed in the Commons, losing 49 of their 57 seats. Olney is now their only female MP.

It wasn’t a particularly good news for Labour either. Jeremy Corbyn’s party lost their deposit, after candidate Christian Wolmar picked up just 1,515 votes, around 3.7 per cent of the total. Reports suggested this was less than the number of Labour Party members in the constituency.

Speaking alongside Olney in the south-west London constituency on Friday morning, Farron said the Prime Minister needed to take notice of this result: “This result might change the direction of British politics. It’s about momentum.”

He added: “If I tell you that nearly a third of Tory voters from the last election who voted Leave in June voted Liberal Democrat yesterday, you will see that this is not just about Remain versus Leave rerun, it’s about people trying to say to Theresa May, we do not like the extreme version of Brexit outside the single market you are taking us down.”

In her victory speech, Olney claimed her win had “sent a shockwave through this Conservative Brexit government”.

She added: “And our message is clear: we do not want a hard Brexit. We do not want to pull out of the single market. We will not let intolerance, division and fear win.”

Olney won 20,510 votes, up 30.4 per cent on the party’s 2015 result, against 18,638 for Goldsmith.

This is the second election Goldsmith has lost this year, after Sadiq Khan beat him in the contest to be Mayor of London.

Speaking at the count result, he said he had stood down because he told his constituents he would if the Government approved the third runway: “This by-election was not a political calculation. It was a promise that I made and it was a promise that I kept.”

The SNP’s Stephen Gethins said the result was a rejection of the Government’s Brexit negotiating position.

“This result demonstrates that there is increasing opposition to a ‘hard Brexit’ driven by Tory right wingers. It will have a damaging impact on jobs and the economy. That is why we have a responsibility to work together as much as we can to protect Scotland and the rest if the UK from the Tories damaging plans through protecting our membership of the single market and the right of EU nationals who have made Scotland their home and contribute so much to remain.”

A Conservative spokesman said: “This result doesn’t change anything. The Government remains committed to leaving the European Union and triggering Article 50 by the end of March next year.”

Andy Wigmore from Leave.eu said the result changed nothing: “The Westminster bubble bed-wetters are spinning that this was a ‘plebiscite on the Government’s Brexit plans’. If they and MPs truly believe they have a mandate to derail Brexit, we urge them to put their money where their mouth is and join us in our call for a General Election.

“Let’s see how far their Europhilia gets them in the 400-plus constituencies that voted to leave.”

Pollster Professor John Curtice cautioned against reading too much into the result: “The polling evidence is that we still have a situation where the country is divided pretty much 50:50, much as it was on June 24.”