The new general secretary of Unison has declared the trade union will end its support for the “destructive right wing of the Labour party” and said any leadership election in 2026 should not swap Keir Starmer for Wes Streeting.

Andrea Egan, who won a decisive victory as a leftwing challenger this week, hit out at Streeting in an article for the Guardian over his handling of the resident doctors’ dispute, saying it was “simply unacceptable for a Labour politician to describe striking workers as morally reprehensible”.

She also called on Starmer to “act now” to stop Palestine solidarity campaigners having “to starve protesting for their basic rights”, in reference to the prisoners on hunger strike.

Her blunt remarks indicate that Unison, a leading union for health and social care workers, is on course for a collision with Streeting and Labour more widely over its approach to industrial action and the Middle East.

Egan will take over the general secretary’s job in January at Britain’s biggest trade union, which is also one of Labour’s largest donors, giving almost £1m this year. She replaces Christina McAnea, who has held the role since 2021 and has been close to Starmer’s government.

In contrast, Egan, a social worker with decades of union experience, is a leftwinger who was expelled from Labour in 2022 after the party said she had shared articles on social media from Socialist Appeal, an organisation banned by Labour.

In her article, Egan suggested a Labour leadership contest was likely in 2026 and warned against the party backing Streeting.

“We will call time on our union’s inexcusable habit of propping up politicians who act against our interests, undermine our fundamental values and make our lives worse,” she said.

“Like colleagues across the movement, I have in recent weeks been appalled by Wes Streeting’s attacks on resident doctors and their union. It is simply unacceptable for a Labour politician to describe striking workers as ‘morally reprehensible’.”

Egan said she would engage open-mindedly with Streeting, but added: “Given the likelihood of a Labour leadership election in 2026, it’s important for me to be clear: swapping Starmer out for Streeting or anyone else from the right wing of the party would be no solution to the gigantic challenges facing the country. What’s needed is a radical change in approach based on the Labour movement’s core values.”

The general secretary elect also indicated she would run the union in a different way, saying: “Some at the top of our movement have contributed to its decline by creating cultures where workers, ordinary members, are consistently disregarded by their own organisations.”

In response to Egan’s criticism, Streeting’s spokesperson said: “Wes is a Unison member delivering with, and for, Unison members – from introducing the first ever fair pay agreements for care workers to ending the use of subcos [wholly-owned subsidiaries] for outsourcing staff and undermining their pay and conditions.

“Wes has never closed the door on resident doctors or the BMA after the 28.9% pay rise he awarded them, and he won’t be doing that now. To act as though the strikes would have no impact on patients or other NHS staff would have been dishonest and irresponsible itself.”

Egan’s comments suggest Unison may in future take an approach closer to Unite, led by Sharon Graham, which has been pressing Labour to stay closer to leftwing values.

Egan said in her article: “It should be clear that putting members first doesn’t mean relegating or turning away from politics. That would be an abdication of responsibility. But it does mean bringing Unison’s support for the destructive right wing of the Labour party to an end.”

The change at the top of Unison leaves Starmer facing concerted pressure on the left from key trade unions on top of the challenge from Zack Polanski’s Green party and Your Party, co-founded by Jeremy Corbyn and Zarah Sultana.

At the same time, Labour recently struck a compromise to water down its employment rights bill, which passed through the Lords this week, after pressure from the right and business groups. The government agreed for workers to qualify for protection against unfair dismissal after six months, instead of the current two years, despite day-one rights being a Labour manifesto pledge. Unions agreed to this climbdown alongside the lifting of the compensation cap for unfair dismissal.

The TUC has praised the bill for banning exploitative zero-hours contracts, bringing in sick pay for all, expanding parental and bereavement leave, repealing Tory anti-union laws, ensuring union access to workplaces, and establishing a social care fair pay agreement.

But it emerged on Friday that Labour Together, a group that helped support Starmer’s leadership, had circulated a discussion paper that suggested scrapping many of the regulations in the recently passed bill to help boost the economy, while floating a new system of employment insurance alongside easier hiring and firing.

A Labour Together spokesperson said: “Labour Together totally supports this week’s Employment Rights Act. This paper was a provocation for private discussion about how to build a political economy centred on technological change and innovation.

“It looks beyond current policy, exploring ideas for spreading innovation and investment outside the south-east and how to support workers through technological change with a bigger welfare state rather than regulation alone. But given the welfare state we inherited from the Conservatives, that is far away. The Employment Rights Act is the best way to give workers much more security now.”