Labour has launched a targeted social media campaign attacking the Green Party ahead of next month’s Gorton and Denton by-election, urging voters not to back the Greens tactically as senior figures warn the contest could fracture into a multi-way fight.

The vote in the Greater Manchester seat is due to take place on 26 February, after former Labour MP Andrew Gwynne resigned from Parliament. Labour’s move comes amid private concerns among some of its MPs that anger at the party and a coalescing anti-Reform vote could hand the Greens a breakthrough under their leader, Zack Polanski.

A Labour video advert, posted online and due to run as a non-skippable placement on YouTube, points to constituency polling showing Labour as the nearest challenger to Reform UK in the seat, while criticising the Greens over policies including the legalisation of drugs. Labour said the advert was viewed by more than 100,000 people in the first hour after it was uploaded.

Sir Keir Starmer, speaking as he prepared to travel to China, insisted the contest was a straight fight between Labour and Reform and argued only Labour could prevent Reform from winning.

“There’s only one party to stop Reform and that’s the Labour party,” he said, describing Reform’s politics as “division” and “toxic division”. He said the by-election would be fought on “Labour values” and on Labour’s record in the constituency.

However, Labour MPs have privately warned that the party risks losing support to the Greens as voters look for an outlet for dissatisfaction with Labour while also trying to block Reform. One MP said: “I will campaign very hard for us to win but the leadership has made it very hard for us.” Another said: “They want us to go and knock doors but we all know it is between the Greens and Reform. It’s Caerphilly all over again.”

The reference was to last year’s Caerphilly Senedd by-election, when Labour lost a seat it had held for more than a century and finished third behind Plaid Cymru and Reform, a result some Labour figures now cite as a warning of what can happen when votes fragment.

The concerns have been sharpened by internal tensions in Labour after the party leadership blocked Greater Manchester mayor Andy Burnham from standing as the Labour candidate, a decision that has caused anger among some local figures and MPs.

Professor Sir John Curtice said the contest could develop into a four-way fight involving Labour, Reform, the Greens and George Galloway, and warned of further pressure on Labour if rival candidates split parts of the electorate.

“This could be a four-way fight between Labour, Reform, the Greens and George Galloway,” he said. “The latter two will compete for and could split the Muslim vote as they did in the seat in 2024, doing a lot of damage to Labour.”

Pollster Lord Robert Hayward said the Greens could be competitive, arguing that a Green win would carry particular significance for Labour in England.

“I can foresee the Greens winning,” he said. “I do think it’s worse for Labour because it identifies an English alternative to Labour on the left which the SNP and Plaid currently fulfil in Scotland and Wales.”

The Green Party has surged in attention since Polanski became leader, and he has previously said he wants the Greens to “replace the Labour Party”. Labour has sought to counter that narrative by circulating polling used in its new advert, which it says shows the Greens in third place in Gorton and Denton behind Reform and Labour.

A Labour insider said the party was responding to what it called misleading claims about the state of the race. “Zack Polanski has spent days trying to peddle false narratives,” the source said. “If he actually read what he’s posting, Polanski would notice that the Green Party are polling in third in Gorton and Denton. The Greens are simply not in the race.”

The same source argued that any shift of Labour votes to the Greens could help Reform. “With Reform picking a candidate who is trying to drive a wedge between communities here in Manchester, we can’t risk the Greens letting Matt Goodwin in through the back door by misleading voters about the state of the race,” the insider said.

Reform announced Matt Goodwin as its candidate on Tuesday.

Labour has also sharpened its broader attack on the Greens in recent days. Last week, Sir Keir described the party as “high on drugs and soft on Putin”, in remarks aimed at challenging Green positions on drugs policy and foreign affairs.

Polanski, whose party is expected to finalise its candidate on Thursday evening, rejected Labour’s framing of the contest and said the by-election was shaping up as a battle between the Greens and Reform, not Labour and Reform.

“It’s Greens vs Reform,” he said. “We’re already seeing that on the doorstep. It’s absolutely game on. We plan to run this campaign like one has never been run before.”

He added: “If you wanted a political story that defines politics in our world right now – this is it. Big money vs community. We’re standing up for the UK versus them standing up for Trump. It’s hope versus hate. We have to win it.”

Labour, for its part, has indicated it intends to fight the campaign on the argument that it is the only credible vehicle for stopping Reform in the seat, and that the Greens’ policy platform will not match local priorities.

“We know anti-social behaviour causes nuisance to people in their local communities,” the Labour insider said. “Therefore locals will be up in arms when they discover the Greens’ plans to legalise all drugs.”

The by-election in Gorton and Denton is set to become an early test of whether Labour can hold together its coalition in urban northern seats as Reform seeks to expand and the Greens attempt to convert rising national support into a parliamentary breakthrough. Further constituency polling and the confirmation of candidates, including whether Galloway stands, are expected in the coming days.