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.
previously told House magazine, external: “I am a fan of Andy. I mean, he’s the King of the North!”