BORIS JOHNSON’S Commons majority dropped to just one today as convicted fraudster Chris Davies was unseated by the Liberal Democrats in a by-election.

Mr Davies lost the mid-Wales constituency of Brecon and Radnorshire by 1,425 votes as the Liberal Democrats took a swing of 12 per cent to win.

Constituents triggered the by-election in a recall petition after Mr Davies’s admission earlier this year to forging invoices for expenses.

The loss will add to Mr Johnson’s challenges as he seeks to push Brexit through Parliament by October 31 and may increase the chances of a snap general election.

With Thursday’s vote taking place eight days after Mr Johnson took office, it is the quickest by-election defeat for any PM since World War II.

Threats from his own MPs to defect to the Lib Dems could cause his majority, propped up by 10 DUP MPs, to be eliminated or go into negative figures.

Bracknell MP Dr Phillip Lee said earlier this week that he would spend the summer parliamentary recess thinking about whether to defect to the Lib Dems and said that around six other Tory MPs were considering it too.

The by-election winner, Welsh Lib Dem leader Jane Dodds, had her campaign boosted by a pro-EU electoral pact with Plaid Cymru and the Green Party who both agreed to not field a candidate.

Ms Dodds said she would seek out Mr Johnson in Parliament to “spell out” the implications of a no-deal Brexit on her new constituency.

She thanked the Greens and Plaid Cymru for their “courageous decision” to back her in the so-called Remain alliance.

New Lib Dem leader Jo Swinson suggested that further pacts will be made in the future with the aim of scooping up more pro-EU votes.

Labour’s candidate Tom Davies came fourth after the Brexit Party’s Des Parkinson. Labour leader Jeremy Corbyn said he was “disappointed” to lose to the tactical vote.

“The Liberal Democrats won it after doing a deal with Plaid Cymru and the Greens,” he said.

“I think that a lot of voters were determined to get rid of the Conservatives and they voted accordingly.

“So we were squeezed, but it’s a place we have not held for a very long time. The area has changed a bit.”

Brecon and Radnorshire was a Labour stronghold after WWII until it was taken over by the Tories in 1979.

It then turned yellow after it was captured by the SDP-Liberal Alliance in 1985 and has switched between Tory and Lib Dem since.

The official turnout was 59.72 per cent. It is the second-highest turnout for a by-election since 1997.