Boris Johnson has been left with a working majority of just one seat after a major defeat in the Brecon and Radnorshire by-election in Wales.
The Liberal Democrat candidate Jane Dodds won by 1,425 votes, in a 12 per cent swing against the government.
She is a child protection social worker who was elected Welsh party leader two years ago.
In her victory speech Ms Dodds challenged the Prime Minister to rule out a no-deal Brexit.
“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,” she 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 defeated Tory candidate Chris Davies, who was forced out of the seat earlier this year after he was convicted for submitting false expenses invoices.
In his concession speech he thanked his team and family “who have had a difficult time over the last few months”.
The Lib Dems secured 43 per cent of the vote, the Conservatives 39 per cent and the Brexit Party 10 per cent. Labour came in fourth with just over five per cent, narrowly avoiding losing its deposit.
The loss leaves Mr Johnson with the support of 319 MPs, while the opposition has 318.