The actor Hugh Grant is among the signatories of a letter urging Westminster party leaders to ban social media for children under 16.

The letter to Keir Starmer, Kemi Badenoch and Ed Davey calls on them to back amendment 94a to the children’s wellbeing and schools bill, ahead of peers voting on amendments on Wednesday.

It is backed by campaigners including the actor Sophie Winkleman and Esther Ghey, the mother of Brianna Ghey, who was murdered by two teenagers in Warrington, Cheshire, in 2023.

The letter states that national polling by the charity Parentkind found 93% of parents think social media is harmful to children and young people.

It says: “No other amendment to the bill on this topic has the same cross-party support or would deliver promptly the change needed to get children off social media.

“While well intentioned, they [other amendments] do not send a clear signal that social media is harmful for children, nor do they do as much to support parents. Amendment 94a is the clearest and most straightforward proposal that meets the scale of the problem and the urgency parents are demanding.”

It goes on: “On behalf of the millions of parents we represent through our polling and campaigning, we ask you to encourage your peers in the House of Lords to support amendment 94a. This is a moment for leadership. Parents are asking for help and parliament now has a chance to provide it to protect childhood. Together we could make a real difference.”

Members of the House of Lords will have an opportunity to have their say on an outright ban when they vote on the amendment brought by Lord Nash.

The amendment has the support of some Labour peers, and if it passes in the Lords it would then be debated in the Commons, where 61 Labour MPs recently wrote to the prime minister urging him to bring in a ban.

Liz Kendall, the technology secretary, reassured MPs on Tuesday that the government’s consultation announced the previous day would take only a few months and the government would have a firm position by the summer.

Proposals for overnight curfews and breaks to prevent “doomscrolling” will form part of the consultation.

Kendall also confirmed that “evidence-based” guidance for parents on appropriate screen time for children aged five to 16 would be produced. Guidance for parents of under-fives is expected to be published in April.

Nash, a former schools minister, said he would press ahead with his attempt to legislate immediately for a ban. He said: “The time for delay and procrastination is over. Without swift action to raise the age limit for social media to 16, we are at risk of a societal catastrophe.

“That is why I am urging all fellow peers to vote for my amendment, which has the backing of peers from every major party, to end the disastrous harm being done to our young people by social media and to give them their childhood back.”

One of those planning to vote for Nash’s amendment is the film director and technology campaigner Beeban Kidron.

She condemned the government’s announcement of a consultation, saying in a statement: “This announcement is an insult to parliament which legislated to make the online world safe, to parents and teachers, and most of all, to children themselves, who repeatedly say that they want to live in an online world where they are free to explore and learn from one another without being exploited by the tech sector,.

“This hastily pulled together announcement is meant to appease the government’s backbenchers, and rescue it from losing votes in the Lords. Keir Starmer pledged to put country before party, but this is the very epitome of party before country. It is not leading; it is not governing.”

Badenoch has already said the Conservative party would introduce a ban for under-16s if it was in power.