More than 10,000 voters in the constituency - one fifth of the electorate - signed what is known as a "recall petition" to have Conservative Chris Davies removed from office.
It is only the third time a recall petition has ever been held in the UK as part of new rules to ensure voters have a say in whether an MP should stay in office when they are convicted of an offence.
Davies, a 51-year-old father of two, was sacked by voters after pleading guilty to two charges of making false claims under the Parliamentary Standards Act 2009 for photographs he had commissioned for his office.
When will the by-election be?
To begin the process, convention says that the Conservative party will "move the writ".
That usually happens within three months of the seat being vacant.
Once that is issued, the by-election will follow between 21 and 27 working days later.
Who is standing?
This section will be updated as candidates are announced.
Despite being sacked by his constituents, Chris Davies has been reselected as the candidate for the constituency.
Born in the Swansea Valley, he was educated at Morriston Comprehensive. He worked as a rural auctioneer and estate agent and managed one of Mid Wales' largest mixed veterinary practices based in Hay-on-Wye.
He was elected to Powys County Council in 2012.
He is a commentator at the Royal Welsh Show and Assistant Honorary Director of Horses.
He's married with two children.
Brecon Town Councillor Tom Davies is the candidate for Brecon and Radnorshire.
He was born and raised in Brecon and has lived in the constituency for most of his life. He was educated at Ysgol y Bannau and Brecon High School.
He read law at Aberystwyth University and is a qualified barrister.
He currently works as a Litigation Officer for a company based in Cwmbran.
He is a keen follower of rugby and football: a season ticket holder for the Scarlets rugby team, as well as following the Welsh football team both at home and across Europe. He also enjoys cooking and has a passion for live music.
The leader of the Welsh Liberal Democrats since 2017, she was born and raised in Wrexham. A Welsh speaker, she studied in Cardiff and trained to be a social worker.
She has previously been a councillor in London. In 2015 and 2017, she was a candidate in Montgomeryshire. She was made the candidate for Brecon and Radnorshire in 2019 after a ballot of members.
She says she wants to "reinvigorate" the party and called on members to "lead the fight on Brexit, climate change, poverty and lonelines.
The Brexit Party has said it will field a "local candidate" but no candidate has yet been named.As I indicated above for Plaid to stand aside for the Liberal Democrats , would be a sacrifice and my problem is that although I have constantly mooted the possibility of a progressive alliance i have doubts it we could trust the Liberal Democrats to use it for their own advantage.
They whilst having a long Pro-EU agenda are banking on this to erase the memory of them being the bag carriers of David Cameron Austerity Program , that in many way saw the growth of the idea that a vote to leave the EU would punish those who have caused misery for many working class people.
If there is going to be one REMAIN candidate in the Brecon and Radnorshire by-election then yea it should be a Liberal Democrat , but how much will they thank or acknowledge those parties who have stood aside?