Britain's pro-European Union Liberal Democrats won the parliamentary seat of Brecon and Radnorshire from the governing Conservatives on Friday.

The move marks a blow to UK Prime Minister Boris Johnson in his first electoral test since taking office, and reduces his working majority in parliament to just one.

Johnson is expecting a showdown with lawmakers in the autumn over his plan to take Britain out of the European Union on Oct. 31 without an exit agreement if necessary.

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LLANELWEDD, Wales (Reuters) - Britain's pro-European Union Liberal Democrats won the parliamentary seat of Brecon and Radnorshire from the governing Conservatives on Friday, a blow to Prime Minister Boris Johnson in his first electoral test since taking office.

The loss reduces Johnson's working majority in parliament to just one ahead of an expected showdown with lawmakers in the autumn over his plan to take Britain out of the European Union on Oct. 31 without an exit agreement if necessary.

Johnson's government already relies on the support of a small Northern Irish party for its wafer-thin majority, with just a handful of rebels in his own Conservatives needed to lose key votes.

"Boris Johnson's shrinking majority makes it clear that he has no mandate to crash us out of the EU," Liberal Democrat leader Jo Swinson, whose party now have 13 seats in parliament, said in a statement following the result.

"I will do whatever it takes to stop Brexit and offer an alternative, positive vision ... We now have one more MP (Member of Parliament) who will vote against Brexit in parliament."

The Brecon vote was triggered when Conservative lawmaker Chris Davies was ousted by a petition of constituents after being convicted of falsifying expenses. Liberal Democrat candidate Jane Dodds won with 13,826 votes.

Davies, who ran again for the Conservatives, came second with 12,401 votes. The Brexit Party came third with 3,331 votes, while the main opposition Labour Party was fourth on 1,680 votes.

The Liberal Democrats had previously held the seat from 1997 until 2015, when it was won by Davies. In the 2017 snap election he held the seat with a majority of just over 8,000 votes.

(Reporting by Rebecca Naden; Writing by Kylie MacLellan; Editing by Elizabeth Piper and Sandra Maler)