After pro-EU parties agreed to back a single candidate, Jane Dodds won Thursday’s by-election by a margin of 1,425 – a swing of nearly 12 per cent against the Tories. Mr Johnson’s first electoral defeat is the swiftest for a new PM since the war.

Ms Dodds pledged to use her vote to help stop a no-deal Brexit and force a second referendum on EU membership, and Lib Dem leader Jo Swinson hinted at further pro-EU alliances across the UK.

Appearing alongside her new MP, Ms Swinson said: “This by-election victory shows that the people of Brecon and Radnorshire not only have a first-class MP in Jane Dodds but they have shown the people of Britain that we can do better than the choice on offer between Boris Johnson and Jeremy Corbyn.”

Liberal Democrat MP Jane Dodds, centre, celebrates with supporters as she wins the seat in the Brecon and Radnorshire by-election at the Royal Welsh Showground, Llanelwedd, Builth Wells. PRESS ASSOCIATION

She added: “I will fight to keep our country in the European Union, and we now have in Parliament one more MP who will fight to make that happen.” The Lib Dem leader said she had exchanged messages with the leaders of the Greens and Plaid Cymru, who stood aside in Brecon and Radnorshire, telling the BBC: “I think there will be more co-operation in future elections.”

Ms Dodds, the Welsh Lib Dem leader, said: “My very first act as your MP when I arrive in Westminster will be to find Mr Boris Johnson wherever he is hiding and tell him loud and clear: ‘Stop playing with the future of our communities and rule out a no-deal Brexit now’.”

She pushed the Conservative candidate, Chris Davies, who was the constituency’s MP until he was ousted in a recall petition after he admitted submitting false invoices for expenses, into second place.

Despite 19 per cent of the electorate petitioning for his recall, the Tories selected Mr Davies to fight to reclaim the seat.

The Lib Dems secured 43 per cent of the vote, the Tories 39 per cent and the Brexit Party 10 per cent.

Fourth-placed Labour narrowly avoided losing its deposit by securing just over five per cent of the vote.

Some Conservative commentators blamed the Brexit Party for splitting the “pro-Brexit vote” – because together their vote share amounted to 49 per cent.

Tory chairman James Cleverly said it was “disappointing to lose a parliamentary colleague” but, trying to put a brave face on the defeat, he said: “What we saw was a very close result in a by-election in which the Lib Dems were expected to romp home comfortably.”

Plaid Cymru’s Westminster leader Liz Saville Roberts said: “The victory of Remain cross-party co-operation and Jane Dodds will give those in Downing Street plotting a snap general election pause for thought, because it shows they cannot take the public for granted.”

Mr Davies won the seat from the Lib Dems in 2015 and secured a majority of 8,038 in the 2017 general election.

But this was overhauled with a swing to the Lib Dems of 11.96 per cent.

The loss leaves Mr Johnson with the support of 319 MPs, including the DUP, which props up his majority, while opposition parties have 318.

Mr Davies’ recall came after he pleaded guilty in March to submitting two false expenses invoices for nine photographs costing £700 to decorate his new office.

He was fined £1,500, ordered to pay £2,500 towards legal costs and told to carry out 50 hours of community service.

In his losing speech, Mr Davies thanked his team and family, “who have had a difficult time over the last few months”.

Boris Johnson hanging by a thread as Lib Dems cut majority to one