Keir Starmer with Andy Burnham at the launch of Labour's election manifesto last year.Andy Burnham has called on Labour MPs to vote down the government’s welfare cuts – despite Keir Starmer’s attempts to buy off his backbench rebels.The Manchester mayor said the prime minister had only performed “half a U-turn”, which did not go far enough.His comments, at an event at the Glastonbury festival, will be seen as a direct challenge to the PM’s authority.The government announced on Friday that it was watering down its plans to cut £5 billion from the welfare budget after a rebellion by more than 120 Labour MPs.It means that those currently in receipt of personal independence payments (PIP) and universal credit will not be affected, but those claiming in the future will receive less cash.Starmer hopes the concessions will be enough to persuade Labour MPs to vote for the government’s reforms in the Commons on Tuesday.