Labour have accused the Reform UK candidate for the Gorton and Denton byelection, Matthew Goodwin, of representing “toxic politics” after he was endorsed by the far-right agitator Tommy Robinson.

The move will be uncomfortable for Nigel Farage, who has consistently kept the parties he leads separate from Robinson, an anti-Islam campaigner and one of the UK’s leading far-right figures.

In a post on X on Friday to his 1.8m followers, Robinson quoted a tweet by Hope Not Hate, which researches and campaigns against far-right politics, adding simply: “Vote for Matt.”

A Reform spokesperson said in response: “We have consistently been clear on this issue. He isn’t welcome in the party.”

But Labour, which is seeking to present itself as the only viable leftwing option for votes against Reform in the Greater Manchester constituency, said Robinson’s intervention showed that “the stakes couldn’t be higher” for the 26 February contest.

“Tommy Robinson’s endorsement of Reform’s candidate shows who he really is and what he stands for,” a Labour campaign spokesperson said. “The Greens simply cannot win this byelection, leaving a stark choice between unity and change with Labour, or division and risk with Reform.

“Only a vote for Labour can stop the toxic politics that Reform and Tommy Robinson would bring to Manchester.”

The Greens, who unveiled their candidate on Friday, argue in turn that Labour’s polling woes and decision not to allow Andy Burnham, the Greater Manchester mayor, to stand in the seat, mean they are the most credible opponent for Reform.

Goodwin, a former academic who is now a GB News presenter and hard-right activist, was a controversial choice to represent Reform in the byelection, particularly after he claimed recently that UK-born people from minority ethnic backgrounds were not necessarily British.

Speaking at an event in Denton on Tuesday at which he was unveiled as the Reform candidate, Goodwin twice declined to answer when asked by the Guardian whether he stood by the views. He had also said: “It takes more than a piece of paper to make somebody ‘British’.”

Robinson, whose real name is Stephen Yaxley-Lennon, first came to notoriety as leader of the English Defence League, an anti-Islam street group based initially around fellow football fans in his home town of Luton.

He has been convicted of criminal offences including assault, mortgage fraud and using a false passport. In 2018 he was made an adviser to Gerard Batten, the then leader of Ukip, prompting Farage to quit the party he had led for years.

Since Robinson’s X account was reinstated under Elon Musk’s ownership of the platform, he has gained prominence, including in the US, presenting himself as a self-styled journalist and campaigner, although the bulk of his content is far-right material targeting immigrants and Muslims.

His increased profile was highlighted last September when a “unite the kingdom” protest he organised was attended by between 110,000 and 150,000 people.