Boris Johnson is set to campaign for the re-election of a convicted Tory expenses cheat in a bid to stop his government’s majority being cut to just one.

The prime minister was expected to visit Brecon and Radnorshire in Wales on Tuesday to urge voters to back disgraced ex-MP Chris Davies in Thursday’s by-election.

Davies was ousted by more than 10,000 constituents in a June recall petition after being convicted over false expense claims he made in 2015.

He was nevertheless reselected by his local party to fight the by-election but is widely expected to lose the seat to the Liberal Democrats, a result which will cut the Tories’ Commons majority to one.

With Johnson likely to face crunch votes designed to stop a no-deal Brexit in autumn, every vote will count in the coming months. 

Labour’s Shadow Justice Secretary Richard Burgon said: “Yesterday, Boris Johnson decided there was ‘no case to answer’ for one of his MPs (Mark Field) who was filmed grabbing a female climate change protester by the neck. 

“Today, he’s backing up one his MPs who’s facing a by-election because local voters sacked him for fiddling his expenses. All of this goes to show that this old Etonian thinks it’s one rule for those who support his hard-right political project and another for everyone else.” 

A Lib Dem source said: “Boris Johnson and Chris Davies are like two peas in a pod. They have misled the public, embarrassed many in their own party and committed to a reckless Brexit plans. They deserve to be with each other; in the dustbin of political history. People deserve better.”

Ahead of the visit, Johnson pleaded with Tory voters in the seat not to switch to Nigel Farage’s Brexit Party.

He said: “The Brexit Party cannot deliver Brexit, only the Conservatives can. Chris Davies is the only candidate you can guarantee will back me so we can leave the EU on October 31.”

He went on: “This by-election is between the Conservatives and the Liberal Democrats, and the Liberal Democrat candidate will do everything they can to stop Brexit. 

“So if the people of Brecon and Radnorshire want to get Brexit delivered on time, they need to vote for my candidate, Conservative Chris Davies.”

Former Lib Dem leader Tim Farron said: “It would appear that Conservative aides have finally dusted off a map, given Boris Johnson the name tag for their candidate and pointed him to Brecon and Radnorshire.

“This last minute quick-dash visit is symbolic of just how much the Conservatives have taken local people for granted. 

“People deserve better than the Tories’ half baked plan to crash the UK out of the EU without a deal and thereby lumping farmers with an eye watering 40% tariff on Welsh lamb.

“By voting Liberal Democrat on Thursday, people across Brecon and Radnorshire will finally get an MP who will oppose this chaos and stand up for farmers and our rural communities.”

In April, Davies was fined £1,500 and sentenced to 50 hours of unpaid work in March after admitting two charges of forging invoices to obtain landscape photographs for his office.

He was given a formal warning by the Conservatives, sparking criticism of Theresa May for refusing to sack him.

Sentencing at Southwark Crown Court in central London, Mr Justice Edis said: “It seems shocking that when confronted with a simple accounting problem, you thought to forge documents.

“That is an extraordinary thing for a man with your position and your background to do.”

The two charges related to the period when Davies was setting up his constituency office following the 2015 snap general election.

He had contacted a photographer in his constituency and bought nine images from him to decorate and display in his constituency office, using his own money to pay the £700 for them initially.

There were two budgets available to him, the Start Up Costs Budget – for office furniture and IT equipment – and the Office Costs Budget, both of which he could claim the full amount from.

But Davies found in February 2016 that only £476.02 was left in the Start Up Costs Budget, with £8,303.75 remaining 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.

The judge 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.”

Davies, who was not seeking to profit financially and was entitled to claim for the pictures, made an “unreserved apology” after his conviction.