The newly-elected MP for Brecon and Radnorshire Jane Dodds has said she would seek out Boris Johnson in Parliament to "spell out" the implications of a no-deal Brexit on her new constituency.
She told ITV Wales, "I will be looking for Mr Johnson wherever he is and I will be saying I want no deal Brexit taken off the table.
"This is unprecedented in terms of, he has lost an election in the first week of his premiership... surely now he has to listen?"
She added, "people have got real concerns about a no-deal Brexit and I want to spell out very clearly to him what the implications would be, which is fairly disastrous if we crash out with a no-deal on October 31st.
"October, November, December are peak times to sell Welsh lamb. There are two issues here, one is economic for farmers; how they are going to cope with 40 per cent tariffs on their lamb exports. Second is mental health. It's a stressful position to be in."
Following Ms Dodds' victory, the prime minister only now has a working majority of just one.
Jane Dodds won the mid-Wales constituency by a margin of 1,425, swinging the vote nearly 12% from the Tories.
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.
The Tory loss will add to the PM's challenges to steer Brexit through Parliament and may increase the chances of a snap general election.
With the vote coming eight days after Mr Johnson was anointed, it is the quickest by-election defeat for any PM of the post-War period.
Welsh Lib Dems leader Ms Dodds immediately challenged Mr Johnson to rule out a no-deal Brexit following her win.
She pushed into second place the Conservative candidate Chris Davies, who was the constituency's MP until he was ousted in a recall petition after he admitted submitting false invoices for expenses.
Mr Davies' recall came after he pleaded guilty in March to submitting two false expenses invoices for nine photographs costing £700 to decorate his new office.
He was fined £1,500, ordered to pay £2,500 towards legal costs and told to carry out 50 hours of community service. In his losing speech, Mr Davies thanked his team and family "who have had a difficult time over the last few months"
Despite 19% of the electorate petitioning for his recall, the Tories selected Mr Davies to fight to reclaim the seat.
Similarly the result was a ''difficult'' one for Labour.
The vote may also be seen as an early victory for the "Remain alliance" of anti-Brexit parties, with Plaid Cymru and the Greens agreeing not to field candidates in order not to split the pro-EU vote.
Ms Dodds thanked those two parties for their "courageous decision".
Plaid Cymru leader Adam Price said the "spirit of co-operation" between the pro-Remain parties had led to Ms Dodds's election, as he called for a second referendum.
The loss leaves the PM with the support of 319 MPs, including the DUP which props up his majority, while opposition parties now have 318.
Who is Jane Dodds?
Jane Dodds is the leader of the Welsh Liberal Democrats.
She's a Cardiff University graduate who is a qualified social worker.
She says she stood for the sat because she thought "we deserve better from our politicians".
Ms Dodds is a remain candidate, who wants a second referendum on Brexit.
She's previously stood in Montgomeryshire (in 2015 and 2017 as an MP) and in 2016 for the Assembly - coming second to the Conservatives all three times.