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 agree to a pact with other pro-EU parties and not take part in the poll.
The current state of parties in Parliament
Number of MPs with each group in the House of Commons
By-elections take place between 21 and 27 working days from the issuing of a writ, resulting in polling day on 1 August.
Opposition MPs had speculated the government did not move the writ earlier in order to avoid a July 25 contest – and spare either Boris Johnson or Jeremy Hunt from having to deal with the by-election on their first full day as Conservative leader.
It is the second parliamentary by-election in Wales this year and could further weaken the government’s position in the Commons.
The Conservatives currently have a working majority of just four over opposition parties, and that depends on the backing of the Democratic Unionists.
In April Labour retained its Newport West seat, following the death of veteran Labour MP Paul Flynn.
Analysis by Mark Hutchings, BBC Wales parliamentary correspondent
Politics, like comedy, is all about timing.
Since the 1980s, representation of the Brecon and Radnorshire constituency has ping-ponged between parties.
Now this summer’s by-election, triggered by a petition of voters, has added spice.
The new Tory leader, to be chosen on 23 July, will barely have had a chance to re-arrange the Cabinet or furniture in Number 10 before this part of Powys delivers the first major test of public opinion.
With the numbers in the Commons so tight, the result could play a significant part in increasing or easing the UK government’s Brexit challenge.
Perfect timing? We’ll see.