Tory MP Chris Davies could face a joint Liberal Democrat and Plaid Cymru threat in a by-election triggered by his ousting in a recall petition.
The 51-year-old was convicted for faking expenses claims after submitting two false expenses invoices for landscape photographs to decorate his new office.
Just over 10,000 people signed the petition, well above the 10% threshold – 5,303 – needed for a recall, meaning there must be a by-election.
The date has yet to be confirmed, but it could be the first electoral test for the new prime minister, as the contest for Conservative Party leader is likely to have finished before it is held, with Theresa May’s successor due to be announced during the week of July 22.
If the Tories lose, the Government’s working majority in the House of Commons would be reduced to just three.
The Lib Dems held the Brecon and Radnorshire seat between 1997 and 2015 and could be aided by pro-Remain Plaid Cymru in order to help secure a People’s Vote in the House of Commons.
A swing of 9.8% would be needed by Lib Dem candidate Jane Dodds to win the seat, which saw a majority of 8,038 for Mr Davies in 2017.
Mr Davies confirmed he intended to stand for Tories in the seat he has held since 2015.
He said: “I am naturally disappointed with the result. What happened was my fault and no one else’s. I sincerely apologise to all the people of Brecon and Radnorshire – and now it is absolutely right that they have their say about whether they would still support me to be there their Member of Parliament in a by-election.
“I hope they do, and look forward to regaining their trust and building on the progress we have made over the last four years.”
He was boosted by support from Foreign Secretary Jeremy Hunt, who is fighting Boris Johnson for the Tory leadership, and tweeted: “Whatever the rights and wrongs of his expenses claim I have only ever know Chris Davies as a decent and honest man and a very diligent local MP. My thoughts with him and his family today”
A Plaid Cymru source said while the party could field its own candidate for the by-election, it is “open” to pacts, agreements, and even a joint candidate for the by-election – adding the Lib Dems would be “the likely choice” to work with.
The party said it would “explore options for how we can work together” with other parties to help deliver a fresh EU referendum.
A spokesman said: “The people of Brecon and Radnorshire have spoken and a by-election will now take place. This will likely be a crucial first test for the incoming Conservative prime minister.
“The two-party status quo in Westminster is breaking down. In last month’s European elections many voters broke the habit of a lifetime by choosing to back pro-People’s Vote parties, rather than the Conservatives or Labour.
“Ensuring those of us who want to see a fresh referendum work together, at every possible level, to deliver this aim is crucial. We will explore options for how we can work together, across party lines, to deliver this in the upcoming Brecon and Radnorshire by-election.”
The Lib Dems have nominated Jane Dodds, leader of the Welsh arm of the party, as its candidate.
She said voters have a “clear choice” between her and the Tories.
She said: “Thousands of residents across Brecon and Radnorshire have taken the chance to demand better than a Westminster politics that fails to take their concerns seriously.
“Now we have a golden opportunity to do things differently. The clear choice in this by-election is between the Conservatives, whose chaos and infighting is letting our communities down, and a better future for our area with the Welsh Lib Dems.”
The Brexit Party, which topped polls in the county of Powys in the EU elections is likely to take votes from the Conservatives.
Party chairman Richard Tice said: “The sheer scale of the vote to force a recall and a by-election shows how strongly the level of dissatisfaction with politics in the country is rising up the agenda.
“The Brexit Party will be contesting the forthcoming by-election. It is clear that people of this county want to change politics for good.”
Welsh Labour, who will field Brecon town councillor Tom Davies, said: “The recall result is another huge blow to Chris Davies’s credibility to serve as Member of Parliament. This is a mess all of his making.
“The Conservatives must not drag their feet. They should listen to the wishes of the voters of Brecon and Radnorshire, call an immediate by-election and let the people hold them to account.”
In April Mr Davies was fined £1,500, ordered to pay £2,500 towards legal costs and told to carry out 50 hours of community service after admitting submitting two false expenses invoices.
Southwark Crown Court heard he bought nine images from a photographer to decorate and display in his new constituency office following the 2015 general election, using his own money to pay the £700 for them initially.
There were two budgets available to him, the Start Up Costs Budget and the Office Costs Budget, both of which he could claim the full amount from.
But the court heard he found in February 2016 that only £476.02 was left in the Start Up Costs Budget, with £8,303.75 in the other.
He then created two fake invoices, so the £700 cost could be split between the two budgets – £450 to the Start Up and £250 for the other.
Sentencing, Mr Justice Edis said the offending was “in quite a different category” to the expenses scandal which brought shame on Westminster a decade earlier, but said: “There was no error here. What you did was done quite deliberately and it must have taken some time to create your fake documents.”