The Green party’s success at winning Muslim votes in Gorton and Denton has sent tremors through Westminster, prompting recriminations and accusations from opposition parties, who sense another major realignment in British politics.

Experts say Hannah Spencer’s unexpectedly wide margin of victory was delivered in part by a significant shift of Muslim voters from Labour to the Greens.

Labour and Reform UK have accused the Greens of playing sectarian politics, highlighting the party’s use of the Indian prime minister, Narendra Modi, in campaign materials, its endorsement by George Galloway and accusations of voter manipulation.

Keir Starmer wrote to Labour MPs on Friday telling them: “[Their] divisive, sectarian politics is a sign that the Greens are not the harmless environmentalists they pretend to be.”

But senior figures within Labour admit that the Greens’ ability to turn out the Muslim vote shows the leftwing party is starting to build the kind of finely tuned political machine on which they themselves have relied for years.

“The Greens were doing a lot of stuff with the mosques, persuading people there they were best placed to defeat Reform,” said one Labour source. “When progressive voters were looking for a party to coalesce around, persuading the networks in the Muslim community that you were the party best placed to win made a huge difference.”

Labour has traditionally relied heavily on Muslim voters. Shortly before the last election, a poll by Savanta found that nearly two-thirds of the UK’s nearly 4 million Muslims intended to vote Labour.

Since the election, anger at the party’s stance on Gaza has been exacerbated by fury at its approach to immigration. Labour MPs say they found many Muslim voters mentioning the name of Shabana Mahmood, the home secretary and one of the country’s most prominent Muslim politicians – and not in a good way.

“Lots of people were angry at Shabana and our approach to immigration in general,” said one MP. The person added they had encountered a lot of hostility to the government’s plans to make it harder for migrants to earn settled status in the UK. “Several people said these rules would not have allowed their parents to make their lives here.”

Accusations from Labour and Reform of sectarian campaigning have focused on a few details. The first is a Green campaign video in Urdu that accused the government of being too close to Modi, the Hindu nationalist prime minister of India, who is a particularly polarising figure for many from Pakistani backgrounds.

The second is the last-minute endorsement of the Greens by Galloway, the firebrand former MP who has previously been accused of running divisive campaigns in areas with high Muslim populations.

Finally, Reform and the Conservatives have highlighted a report by election observers warning of widespread “family voting”, where one member of a family accompanies another into the ballot box with the intention of influencing their vote. The Democracy Volunteers report does not provide any detail on the identities of those its observers saw apparently colluding on votes.

Matt Goodwin, Reform’s candidate for the seat, claimed a “dangerous Muslim sectarianism” had emerged and said there was “one general election left to save Britain”. Nigel Farage made a broad claim of “serious questions about the integrity of the democratic process in predominantly Muslim areas”.

Those accusations have prompted anger among prominent British Muslims, who accuse Reform of trying to deny the legitimacy of an entire community by linking the “family voting” report to the wider legitimate effort to court Muslim voters. They point out that shaping messages to a particularly demographic group is a universal feature of political campaigns.

Wajid Akhter, the secretary general of the Muslim Council of Britain, condemned what he called “gutter” rhetoric from “a desperate political class not willing to give ordinary British voters the dignity they deserve”.

Shaista Gohir, who leads the national charity Muslim Women’s Network UK, said: “Muslims have every right to vote for the party that listens to them and is most aligned with their issues just as many voters now seem to be aligned with the Reform party.”

It is not the first time concerns have been raised about the way in which family and religious networks can influence votes among British Muslim voters. A report by academics at Manchester and Liverpool universities in 2015 found: “These networks tend to be reciprocal, and are hierarchical and patriarchal, which may undermine the principle of voters’ individual and free choice.”

Green officials point out that voter fraud tends to be most prominent in postal ballots, and that on this occasion Labour won the postal vote. Experts also say any voter manipulation is unlikely to have been so widespread as to call into doubt Spencer’s 4,402-vote majority.

A Green spokesperson said: “The scale of our victory shows that the Green party has picked up substantial support in all parts of the constituency, in all areas, among all people. It was a victory for unity over division, for hope over hate.”

One thing Labour and Green sources agree on is that in an election where progressive voters were seeking credible information for how to block Reform, Muslim community networks proved vital messaging tools. They add that this could be even more important at the next general election if it becomes a de facto referendum on Reform and its leader, Farage.

“At some point you reach a tipping point where Labour’s most traditional voters realise they can depart the party en masse and still keep out Reform,” said one Labour source. “That’s the real danger for us at the next election.”