Keir Starmer sought to ward off the first rebellion of his premiership with the launch of a new cross-government taskforce to tackle child poverty, as backbench Labour MPs were preparing to support calls for the controversial two-child benefit cap to be scrapped. Starmer’s first king’s speech contained no specific measures to address child poverty, angering dozens of MPs on his own benches, given many of them have constituents suffering as a result of the policy introduced by the former Conservative chancellor George Osborne. The Guardian understands the SNP has officially tabled an amendment to the king’s speech to scrap the policy. The party’s amendment has received cross-party support from the Green party, the Social Democratic and Labour party, Plaid Cymru, Alliance and independent MPs including Jeremy Corbyn. A number of Labour leftwingers had signalled their intentions to back an amendment by the MP for Liverpool Riverside, Kim Johnson, on Wednesday night, which is expected to include a first steps focus for the government to scrap the policy, then introduce free school meals, raise child benefit and improve universal credit. The prime minister had sought to placate the rebels in his own party, by launching the taskforce, which was first called for by the former prime minister Gordon Brown in May, with the strength of feeling over the two-child limit being widespread across the parliamentary Labour party. In a rare intervention, Sarah Owen, the MP for Luton North, told the Commons that more than 45% of children in her constituency are living in relative poverty, and she asked Starmer for assurances that he will address the issue and take it seriously. New Labour MPs including Torsten Bell have previously called for the policy to be ditched, but given how early this parliamentary test has come, may avoid committing to back an amendment in a bid to avoid the ruthlessness of Labour discipline seen in opposition. A new child poverty unit will be set up in the Cabinet Office, and will report to the taskforce, co-chaired by the work and pensions secretary, Liz Kendall, and the education secretary, Bridget Phillipson. Expert officials from across government will work with leaders from the charity sector to consider how the government can “use levers” to improve children’s experiences and chances at life. Each of the organisations which attended a meeting with Kendall on Wednesday morning – Save the Children, Action for Children, Barnardo’s, TUC, End Child Poverty, the Resolution Foundation and Unicef – have in the past called for the two-child limit to be urgently scrapped. Paul Carberry, the chief executive at Action for Children, said: “I was pleased to meet with the secretary of state for work and pensions, Liz Kendall, today to discuss how to urgently help lift children out of poverty. It’s right that the new taskforce and Child Poverty Unit works across government to look at the wider drivers and solutions to child poverty. He added: “A strategy for tackling child poverty must start with abolishing the cruel and unnecessary two-child limit. This would immediately lift around 300,000 children out of poverty.” Tom Pollard, the head of social policy at New Economics Foundation thinktank, whose former CEO Miatta Fahnbulleh is now a New Labour MP said: “It’s welcome to have a government making an explicit commitment on child poverty but it is hard to see beyond the pressing need to get rid of the two-child limit. It needs to go as soon as possible.” At least one in four children live in absolute poverty, and 1.6 million children live in families affected by the two-child benefit limit. The policy means low-income parents are denied key benefits, including universal credit, for their third and any subsequent children born from April 2017. John McDonnell, the Labour MP and former shadow chancellor, said: “It’s the timing of the announcement that they have been forced into, bringing it forward and so prominently. But this is just a repeat of what was said and so it is not enough. So the pressure for scrapping the two-child benefit cap will continue.” Launching the taskforce, Starmer said ministers would “leave no stone unturned” in their efforts to give every child the best start in life. “For too long children have been left behind, and no decisive action has been taken to address the root causes of poverty,” he said. “This is completely unacceptable, no child should be left hungry, cold or have their future held back.” Weeks before he won the election, Starmer told reporters he was “not immune” to the powerful argument for scrapping the policy, but refused to set out a timeline for when he would ditch the scheme introduced by Osborne. Stephen Flynn, the SNP leader in Westminster, said: “I urge Labour MPs in Scotland to vote for the SNP amendment. If they fail, they must take responsibility for the child poverty their Labour government is causing in Scotland. “Scrapping the two-child cap is the bare minimum required. Substantial action is needed to eradicate child poverty, which is why SNP MPs are also pressing the Labour government to match the Scottish child payment by raising universal credit by £26.70 per child, per week.”