Former Labour cabinet minister Alan Milburn has accepted a role as social mobility tsar to the coalition government, it has been confirmed.
Mr Milburn undertook a major study on social mobility before standing down from Parliament at the last election.
The former Darlington MP has now taken a post advising on improving the life chances of the least well-off.
Labour leadership contender Andy Burnham said he had put his own ego above the people he used to represent.
'Closed shop mentality'
In government, Mr Milburn was considered an arch Blairite who repeatedly clashed with the then chancellor, Gordon Brown.
But when Mr Brown became prime minister, he brought him back to head a new Social Mobility Commission.
Mr Milburn's report published last July contained proposals on breaking down the "closed shop mentality" of the professional classes.
He was said to be disappointed his recommendations were not taken forward by the former Labour government.
The BBC's Norman Smith said Mr Milburn had been told he would work in an unpaid capacity as "an independent assessor" free to criticise the government.
Our correspondent said the appointment was "quite a coup" for Prime Minister David Cameron, as it could be seen to make it harder for Labour figures to criticise government proposals.
Deputy Liberal Democrat leader Simon Hughes welcomed the appointment of Mr Milburn, who comes from a working class background.
He said: "Alan Milburn has personal experience of how to work your way up from difficult beginnings.
"I think we have got to be non-partisan and non-tribal about these things and if good people are willing to work for the government, whatever their background, they should be welcomed."
But Mr Burnham, former health secretary, said: "Joining a government that is taking hope away from young people, whose first action was to scrap the Future Jobs Fund, is no way to increase social mobility.
"And it is no way to treat the good people who voted for a Labour candidate and who now, more than ever, need Labour to stand up for them."
Mr Milburn, who stood down as MP for Darlington in north east England in May, was also criticised by Lord Prescott.
Labour's former deputy leader wrote on his Twitter page: "So after Field & Hutton, Milburn becomes the 3rd collaborator. They collaborated to get Brown OUT. Now collaborating to keep Cameron IN."
Mr Prescott was referring to former welfare minister Frank Field, who is devising an anti-poverty strategy, and former work and pensions secretary John Hutton, who is reviewing public sector pension provision.
Another Labour MP, Graham Allen, is also doing work for the coalition on early intervention for children from disadvantaged backgrounds.
Deputy Prime Minister Nick Clegg is due to make a speech on social mobility next week on the coalition's 100th day in office.