Alan Milburn, the Secretary of State for Health, insisted the NHS would continue to be free and comprehensive last night, as he attempted to head off union opposition over claims the Government was watering down the founding principles of the health service.

He issued a statement on the eve of Labour's national policy forum meeting today after The Independent revealed that a party policy document gave a vision of an NHS "largely comprehensive" and "overwhelmingly free".

Trade unions will voice opposition to any move to water down the founding principles of the NHS, despite furious claims by ministers that they would not increase charges. Senior Labour backbenchers also said the paper amounted to a watering down of the principles of the NHS.

But Mr Milburn said: "The NHS will remain based on clinical need and not ability to pay; as free at the point of use and at least as comprehensive as it is today. There will be no change in Labour's commitment to the principles of the NHS, nor the comprehensive nature of its provision. The only party seriously considering new charges for NHS services is the Conservative Party."

Members of the 182-strong Labour national policy forum will debate the document today at the party's Millbank headquarters in London. But it was criticised by Dave Prentis, general secretary of Unison. He said: "If the words mean that there will be more charges, for example to see your GP, or more privatisation then there will be an unholy war in the Labour Party and we will fight tooth and nail to get them removed."

The Labour MP Dr Ian Gibson added: "I think there is a process going on of softening everybody up to aspects of privatisation being introduced into the health service."