All four members of the board of the government's social mobility commission have quit over the lack of progress towards a "fairer Britain".
Alan Milburn, chair of the commission, said in his resignation letter to Theresa May: "I remain deeply committed to the issue, but I have little hope of the current government making the progress I believe is necessary to bring about a fairer Britain.
"It seems unable to commit to the future of the Commission as an independent body or to give due priority to the social mobility challenge facing our nation."
But he said that "individual ministers such as the Secretary of State for Education have shown a deep commitment to the issue."
In his letter to the prime minister, Milburn added: "It has become obvious that the government as a whole is unable to commit the same level of support.
"It is understandably focused on Brexit and does not seem to have the necessary bandwidth to ensure that the rhetoric of healing social division is matched with the reality.
"I do not doubt your personal belief in social justice, but I see little evidence of that being translated into meaningful action."
'Champion of the cause'
Mr Milburn took up his role with the commission – an independent body that monitors progress towards improving social mobility in the UK – in July 2012.
Speaking on the Andrew Marr show this morning, Mr Milburn suggested that education secretary Justine Greening wanted to reappoint him as chair.
He said: "It wasn’t that nobody was listening. Justine Greening, who will be on your programme later today, is a champion of the cause and I know wanted to secure my reappointment as chair of the commission.
"She failed in doing so and I have decided that I am not going to reapply for the job and frankly neither are the three other commissioners."
Speaking to Marr today, Greening refused to say whether Milburn's claim was true.
But she said: "He has done a fantastic job but his term had come to an end. And I think it was about getting some fresh blood into the commission."
It is understood the government will look for a new chair in 2018.