Over 400 doctors, including GPs and consultants, signed the letter which calls for Mr Cameron to drop the Bill and ensure that ‘the damaging effects of reforms are reversed with immediate effect’.

Signatories included retired GP Dr Kailash Chand, BMA council member and GP Dr David Wrigley and GPC negotiator Dr Peter Holden.

The letter warned that the Bill would lead to poorer patient care and the future privatisation of the NHS.

‘The Bill will enhance competition on the basis of cost and not quality, it will pit the different parts of what should be a coordinated system against one another, and it will lead to fragmentation of the NHS within England,' the letter said.

The doctors also raised concerns over the health secretary’s diminished responsibility for the NHS, and expressed fears that the roll out of the any qualified provider policy would ‘see costs climb uncontrollably at the cost of quality of care’.

The letter comes as Liberal Democrat MP Simon Hughes called for health secretary Andrew Lansley to step down.

Speaking on the BBC’s Andrew Marr show on Sunday Mr Hughes said: ‘My political judgement is that, in the second half of the parliament, it would be better to move on.’

The Bill is currently being discussed in the Lords as part of the report stage of its process through parliament.

• NHS reforms: Interactive Health Bill timeline