Teaching unions have written to the prime minister asking him to scrap the two-child benefit cap ahead of Labour’s annual conference, which begins in Liverpool on Sunday.

In a letter to Keir Starmer, the unions described the policy as “cruel” and said it must be fully abandoned as part of the government’s delayed child poverty strategy, expected to be published before Christmas.

The policy was introduced by the Conservatives in 2017 and means most families can only claim certain benefits – such as universal credit and tax credits – for their first two children. The Resolution Foundation thinktank estimates that removing the cap could lift 470,000 children out of poverty.

The National Governance Association, the National Association of Head Teachers, the Association of School and College Leaders, the National Education Union, NASUWT and Unison warned in the letter of the “critical need” to scrap the policy.

The letter read: “As school leaders, headteachers, governors, teachers and support staff working in schools and academies across England, we are writing to you about the forthcoming government child poverty strategy and the critical need for it to fully scrap the two-child limit policy.

“This poverty producing policy is harming the lives of hundreds of thousands of children and young people in our classrooms, and we are calling on government to put an end to this this autumn.

“No child deserves to live in poverty, full stop. But the educational impact of the poverty that continues to rip through our communities and schools cannot be overstated.

“Poverty is having an adverse impact on children’s ability to learn, with children living in low-income households doing worse on average than their peers at every milestone, and it is making it increasingly hard for educators to carry out their core roles, with 79% of school staff in all roles reporting this.

“The government cannot claim an ambitious child poverty strategy while any part of this policy remains in place.

“We work tirelessly every day to protect children from the harms of poverty, but we come together on behalf of the teachers, school leaders, governors and support staff we represent to ask government to meet us in the middle. We need bold action that addresses poverty at home, to ensure all children can thrive at school.”

Speculation is growing that the cap could be removed. Last week, the education secretary, Bridget Phillipson, said that scrapping the policy was “on the table”. Phillipson, seen as Downing Street’s favoured candidate for Labour’s deputy leadership, described the cap as “spiteful” and said it had “punished and pushed children into hardship”.

More than 100 Labour MPs recently signed a letter to the chancellor, Rachel Reeves, urging her to change the policy in the budget. On Friday, Labour restored the whip to John McDonnell and Apsana Begum, two backbench MPs who rebelled over the policy last July. Writing on X, McDonnell, a former shadow chancellor, said: “I hope this is a signal the government has decided to scrap the cap.”

A government spokesperson said: “Every child, no matter their background, deserves the best start in life. That’s why our child poverty taskforce will publish an ambitious strategy to tackle the structural and root causes of child poverty.

“We are investing £500m in children’s development through the rollout of Best Start family hubs, extending free school meals and ensuring the poorest don’t go hungry in the holidays through a new £1bn crisis support package.”