A by-election to choose an MP for Brecon and Radnorshire is expected to be held on 1 August.
Tory chief whip Julian Smith moved the writ in the Commons to trigger the contest after Conservative MP Chris Davies was unseated by a petition.
More than 10,000 people signed the recall petition following Mr Davies’s conviction for a false expenses claim.
He had been the area’s MP since 2015, when he ousted incumbent Liberal Democrat Roger Williams.
His majority then was 5,102, which he increased to 8,038 at the 2017 general election.
Mr Davies has been re-selected for the contest. He will face Jane Dodds, Welsh Liberal Democrat leader, and Labour’s Tom Davies in the election.
The Brexit Party is also expected to stand.
Plaid Cymru has been considering whether it should