The polls have closed in the three-way battle for Gorton and Denton in south-east Manchester after one of the most unpredictable byelections in years.

The Green party leader Zack Polanski said before voting that his party was “neck and neck” with Reform UK to overturn Labour’s 13,000-vote majority, and that Labour will need to “search their conscience” if Reform UK wins.Keir Starmer’s party had targeted left-leaning voters in the Greater Manchester seat with claims that only Labour can see off Nigel Farage’s Reform, saying that a vote for the Greens was “in effect, a vote for Reform”.

Labour’s strategy of claiming the Greens could not win had echoes of the disastrous Caerphilly byelection in October, which the party lost to Plaid Cymru despite telling voters repeatedly: “Only Labour can beat Reform.”

Labour is defending a 13,413-vote majority in Gorton and Denton, where nearly 80% of voters backed a party on the left at the 2024 election. The result will be declared at about 4am on Friday.

Angeliki Stogia, a councillor, was selected as the Labour candidate after Andy Burnham was prevented from standing. The academic turned GB News presenter Matt Goodwin – who has faced criticism for his comments on women, Muslims and British citizenship – stood for Reform UK. Hannah Spencer, a Trafford councillor and plumber by trade, is the Green party candidate.

Prof Will Jennings, of the University of Southampton, said the contest was too close to call and that in Britain’s new fragmented politics “anything can happen”. He said a Labour defeat would be “terminal” for No 10’s strategy to try to appeal to right-leaning voters, which has alienated its core progressive supporters.

“It would be a symbol of the failure of that strategy and the end point for it,” said Jennings. “The worst-case scenario for Labour is coming third behind Reform and the Greens, not least because of the decision to stop Andy Burnham from standing.”

A Labour victory would “staunch that sense of inevitability of the end of Starmer”, he added, and potentially mark a turning point for a government eight points behind Reform in the polls and facing a resurgent Green party.

However, any relief for Starmer would be short-lived as Labour is expected to suffer heavy losses when voters across England, Scotland and Wales go to the polls in the local and devolved elections in 10 weeks.

On a visit to the constituency on Monday, Starmer had described the Greens’ plan to legalise drugs as “disgusting” and claimed it would turn parks and playgrounds into “crack dens”.

Polanski said Starmer’s visit “felt very much like spoiler behaviour” because it suggested Labour was confident of winning when, he claimed, that was not the case. “I think the Labour party will have to search their conscience if they’ve allowed the Reform party to win.”

Polanski accused Labour of sinking to “a new low” with an attack advert on social media showing a green syringe alongside the words: “Heroin, crack cocaine, spice. Green party says YES.”

“It’s the last desperate gasp of a Keir Starmer Labour government,” the Green party leader said.

The byelection was triggered by the resignation of Andrew Gwynne on health grounds in January. The former MP was under investigation by parliament for offensive messages he sent in a WhatsApp group of local Labour figures.