Andy Burnham will face Reform UK’s Robert Kenyon in next month’s crucial Makerfield byelection in a clash that could change the course of British politics for years to come.

Reform are billing Kenyon, a plumber and army reservist who contested the seat just outside Wigan in the 2024 general election, as a local champion taking on a professional politician who is using the seat for his own advantage.

Kenyon, however, faced immediate scrutiny of his social media activity. Deleted posts on X show he appeared to cast doubt on the efficacy of a vaccine, interacted with a Dutch far-right influencer and praised Donald Trump. The Conservatives also asked why Kenyon’s account had been suspended.

Burnham, the Greater Manchester mayor, was selected by Labour’s national executive committee. No other candidates were on the shortlist despite others having applied. The byelection is expected to take place on 18 June.

Burnham has been open about wanting to return to Westminster to change Labour’s direction at the national level, and a leadership bid against Keir Starmer is widely assumed if he wins. He said in a statement that he was humbled to have been selected and promised to put the spotlight on what he said were neglected parts of the UK such as Makerfield.

Announcing Kenyon as Reform’s candidate, the party leader, Nigel Farage, characterised the byelection as a “David versus Goliath battle”.

In a video posted by Reform, Kenyon took aim at Burnham, saying he was using Makerfield as a stepping stone. This is likely to be an attack line in the weeks ahead despite Burnham’s family home being nearby.

“Labour and probably the other parties have got career politicians. They go to private school, to university, they get a job at a thinktank or they are an assistant to an MP and then before you know it they are parachuted into somewhere they have never even visited to stand as an MP,” he said.

Kenyon, who was born in Makerfield and previously worked as a specialist NHS technician in Lancashire, came second in the 2024 election, 5,399 votes behind Labour’s Josh Simons, who announced last week he would give up his role so Burnham could launch his attempt to return to Westminster.

Archived copies of Kenyon’s X posts include one in which he replies to an NHS account questioning its claim that a vaccine has 90% effectiveness, asking: “How many people were tested and what is the protection rate for people who hadn’t had the vaccine?”

In the aftermath of the murder of three young children in Southport, Kenyon replied to a post by Eva Vlaardingerbroek, a prominent Dutch influencer who was among 11 far-right activists banned from the UK before a rally last weekend. Kenyon asked her: “Any description of the attacker?”

He also expressed support for the US president in a number of posts, telling another X user: “Trump is very popular, just not in your liberal circle.”

Labour and Reform are favourites for the byelection, but they face challenges from the left and right. Restore Britain, the openly far-right party set up by the former Reform MP Rupert Lowe, has selected the local businesswoman Rebecca Shepherd as its candidate and deployed targeted Facebook adverts in Makerfield on Tuesday. The Greens are expected to pick a candidate on Wednesday.

Starmer chaired a cabinet meeting on Tuesday morning, the first since Wes Streeting, another expected leadership rival, resigned as health secretary.

The prime minister later used a Downing Street event to hail the impact of the Renters’ Rights Act, which gives new protections to tenants. “We had to fight for this,” he said. “This is not a dry piece of legislation that just sits on the statute books … it makes a real impact for renters up and down the country.”

Starmer has refused to consider the idea of setting out a timetable to quit, saying he hoped to lead the party into the next election.

Mark Drakeford, who was the Labour first minister of Wales from 2018 to 2024, said on Tuesday that he wanted Burnham to replace Starmer.

“I think Keir Starmer should set out a timetable for a change in leadership of the United Kingdom Labour party and the United Kingdom itself,” he told Channel 4 News.

“He is a decent man who works extremely hard every day to do his very best, but a combination of circumstances and capacity mean that it hasn’t worked out, and it’s time to recognise that and make a plan to move ahead.”