PRIME minister Boris Johnson has been dealt a significant blow after the LibDems won the Brecon and Radnorshire by-election, leaving the Tories with a working majority of just one.

Jo Swinson's party won the mid-Wales constituency with a margin of 1425 – a 12% vote swing from the Tories.

The by-election was Johnson's first major challenge as Tory leader and has now become his first major defeat.

The loss will add to the PM's list of challenges as he attempts to steer Brexit through Parliament and could increase the chances of a snap General Election before the UK leaves the EU.

The National: Jane Dodds, centre, celebrated her winJane Dodds, centre, celebrated her win

Jane Dodds, who won the Brecon and Radnorshire seat, said: "People are desperately crying out for a different kind of politics. There is no time for tribalism when our country is faced with a Boris Johnson Government and the threat of a no-deal Brexit.

"So 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'."

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

Fourth-placed Labour narrowly avoided losing its deposit by securing just over 5% of the share.

READ MORE: Labour less popular than the Brexit Party in Scotland

Some Conservative commentators blamed the Brexit Party for splitting the "pro-Brexit vote" - as together their vote share amounted to 49%.

Others pointed out that the new PM had set a new record.

Johnson has lost a seat in only 11 days, faster than any PM since Asquith in 1908.