Left-wing candidate Andrea Egan has been elected as the new leader of Unison, the UK’s biggest trade union, in a blow to Sir Keir Starmer.

Ms Egan, who was expelled from the Labour Party three years ago, beat the union’s current general secretary Christina McAnea, an ally of the prime minister.

Following her election, Ms Egan promised to “stand up to any employer, politician or cabinet minister who acts against our interests”.

During her campaign, she had pledged to hold Labour’s “feet to the fire” and refuse to write the party “blank cheques”.

Congratulating Ms Egan on her victory, Sir Keir said he looked forward to working with her.

The PM also praised Ms McAnea’s “outstanding contribution to the Labour and trade union movement”.

“She played an important role in securing the landmark Employment Rights Bill and her work has ensured this Labour government will bring in a fair pay agreement to boost the wages of social care workers,” he added.

With more than 1.3 million members working in sectors providing public services, including education, local government, the NHS and the police, Unison is the UK’s biggest trade union.

Ms Egan won just under 60% of the vote, although turnout was low, with fewer than 100,000 of the union’s members taking part.

She will begin her five-year term in January.

In a statement following her victory, Ms Egan said: “This result means ordinary Unison members are at long last taking charge of our union.

“We will put faith in members’ decisions and stand up to any employer, politician or cabinet minister who acts against our interests.”

Ms Egan’s election was welcomed by former Labour leader Jeremy Corbyn and his new left-wing group, Your Party.

A Your Party spokesperson said: “Andrea’s victory on an anti-austerity ticket shows that working-class people won’t take more cuts and more decline.

“Unison members have said no to Starmer’s crumbs off the table, no to chronic low pay, no to underfunding and no to privatisation.”

Ms Egan, who has served as Unison’s president since 2022, has criticised welfare cuts, backed a wealth tax and called for a reversal of “austerity” in schools and local government.

During her campaign, she pledged to launch a review of Unison’s relationship with Labour to ensure “value for money” and advance the union’s policies.

Unison is one of Labour’s biggest financial backers, donating £1.49m during last year’s general election campaign.

Ms Egan has said she will not take the £181,000 salary on offer for Unison’s general secretary, instead receiving the wage of social worker, her current career.

Ms McAnea’s defeat weakens Sir Keir’s strong grip on the party’s ruling National Executive Committee (NEC), which has two representatives from Unison.

“It makes [controlling] the NEC harder,” a Labour source said.

Ms McAnea was one of the PM’s most solid and vocal supporters amongst trade union leaders and is close to Sir Keir’s chief of staff Morgan McSweeney.

In contrast, Ms Egan is to the left of Labour and was expelled from the party in 2022 for sharing online articles from a Marxist group.

She could apply to rejoin in 2027, after a five-year block.

“The impact on Labour and Keir is huge,” said one insider.

“But there’s not much the party or government could have done. The campaigning work was clearly not done by Christina’s team with such low turnout.”

Another Labour source said frustration with the state of the economy would have encouraged some Unison members to back a new leader.

“The general malaise and low real incomes will have mobilised the dissenters,” they said.

“It can’t help but be a reflection on Keir and Rachel [Reeves] but it’s not primarily that.”

The UK’s two biggest unions – Unite and Unison – will now be led by left-wing leaders who are critical of the government.

Some in Labour believe this could put pressure on the bosses of moderate unions like USDAW and GMB to be more vocal in their criticism of government policies.

Ms Egan’s election and Unison’s influence on the NEC could also make it easier for Greater Manchester mayor Andy Burnham to be selected to fight a by-election and potentially become an MP.

The NEC has a key role in shortlisting candidates for parliamentary seats.

Ms Egan, who is from Bolton, previously told House magazine: “I am a fan of Andy. I mean, he’s the King of the North!”