The new British Prime Minister Boris Johnson has suffered his first major defeat.

Hid Conservative Party lost a by-election in Wales, after the Liberal Democrats took the Brecon and Radnorshire seat.

The result is a significant blow, as his working majority in the UK parliament has now fallen to just one.

Jane Dodds, the Lib Dems' leader in Wales, won with 43.5% of the vote, ahead of former Conservative MP Chris Davies on 39%.

He triggered the by-election following his conviction for submitting false expenses.

He pleaded guilty in March to submitting two false expenses invoices for nine photographs to decorate his new office.

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

Conservative Party candidate Chris Davies speaks on stage as Liberal Democrats MP Jane Dodds looks on after winning the seat in the Brecon and Radnorshire by-election in Wales | Image: Ben Birchall/PA Wire/PA Images

The British Labour Party was beaten into fourth place by Nigel Farage's Brexit Party on 10.5%.

The loss will add to Mr Johnson's challenges to steer Brexit through the House of Commons, and may increase the chances of a snap general election there.

After the result was announced, Ms Dodds said her first act as an MP would be to find Mr Johnson and tell him to rule out a no-deal Brexit.

She 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'."

Liberal Democrat MP Jane Dodds (centre) celebrates with supporters as she wins the seat in the Brecon and Radnorshire by-election in Wales | Image: Ben Birchall/PA Wire/PA Images

After 11 days in power, Mr Johnson has set a record for the fewest number of days taken for a new British prime minister to lose a seat at a by-election.

He now has the support of 319 MPs, including Northern Ireland's DUP which props up his majority, while opposition parties have 318.

Following the by-election result, newly-elected Lib Dem leader Jo Swinson said: "Boris Johnson's shrinking majority makes it clear that he has no mandate to crash us out of the EU."

The turnout for the Brecon and Radnorshire by-election was 59.72% - the highest for a by-election in the current parliament.