Conservative MP Chris Davies lost his seat in the House of Commons after 19% of registered voters in his constituency signed a petition to remove him.
Mr Davies, 51, was convicted for faking expenses claims after submitting two false expenses invoices for landscape photographs to decorate his new office.
Some 10,005 people signed the petition, well above the 10% threshold - 5,303 people - needed for a recall. It is the third time a recall petition has been triggered since legislation introduced the measure in 2015, and the second time it has resulted in the removal of an MP.
A swing of 9.8% is required for the Liberal Democrats to take the seat. The latest polling on friday put the Lib Dems in second nationally, ahead of the Conservatives.
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Mr Davies has been MP for Brecon and Radnorshire since the 2015 general election, where he beat incumbent Lib Dem Roger Williams with the constituency's largest majority since 1983.
In 2017, he held a majority of more than 8,000 in a constituency where just over 41,000 people voted.
But in April he 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.
The recall petition opened on May 9 and Mr Davies's constituents had six weeks to sign it before it closed at 5pm on June 20. Mr Davies is permitted to stand in the by-election.
Jane Dodds, the Lib Dem leader in Wales and candidate in Brecon and Radnorshire 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.”
A spokesman for Welsh Labour said: "The recall result is another huge blow to Chris Davies' 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."