An historic agreement between the NHS and the independent sector was signed yesterday by Health Secretary Alan Milburn - who immediately came under fire from backbench Labour MPs.

The so-called concordat will allow NHS patients to be treated in private hospitals when there are no free beds or operating space available in the health service.

The Government was accused by some of its own backbenchers of privatising the health service.

But ministerial sources described the concordat as "breaching the ideological barricades" to using private sector resources.

Mr Milburn denied the move was "privatisation by the back door", insisting it would merely allow more NHS patients to be treated more quickly.

However, David Hinchliffe, the Labour chairman of the Commons health select committee, said the concordat was "wrong".

He said: "It is wrong of the Government to get into bed with the private sector which has for over 50 years of the NHS constantly tried to undermine the concept of state health care."

Labour MP Tony Benn said the agreement amounted to privatisation of the NHS and erosion of the welfare state.

He said: "This is a very serious move. It is the privatisation of the NHS.

"The NHS is run down for lack of money, so you bring in the private sector. Once you have done that, why not make it all private?"

Paul Marks, of Unison, the UK's largest union, said: "Of course we want to see waiting lists reduced, but this use of the private sector is stop gap politics."

The deal will mean that when there are bed shortages, or lack of operating space in an NHS hospital, patients will be treated in a private hospital, using NHS doctors and nurses, with care still free at the point of delivery.