Medical inspectors would be appointed in every NHS trust to police the performance of doctors under proposals published yesterday for the biggest shake-up of medical regulation for 150 years.

Doctors would be subject to tougher five-yearly checks on their performance and for the first time the notion of a "good doctor" would be defined and practitioners tested against that standard. But the cost of the new scheme would double the cost of current arrangements from £78m to £150m, and the proposals are likely to put the medical profession on a collision course with the Government. Medical organisations warned they would be unfair and unworkable.

Sir Liam Donaldson, the Government's chief medical officer, who published the proposals in a report Good Doctors, Safer Patients, said the aim was to avoid a repeat of the case of Harold Shipman, the GP whose murderous activities went undetected for decades, and of a string of other medical scandals in which patients had been harmed, abused or put at risk because of failures of medical regulation.

The biggest loser would be the General Medical Council, set up in 1858, which would be stripped of many of its powers, including its role as both prosecutor and judge of doctors accused of misconduct. But its influence would be extended through the new medical inspectors, a network of GMC affiliates who would be practising doctors appointed to act as its eyes and ears, monitoring doctors' performance at local trust level.

The most serious cases of misconduct, currently tried by the GMC to a criminal standard of proof ("beyond reasonable doubt"), would be dealt with by new three-person tribunals operating a lower civil standard of proof ("the balance of probability") - which provoked a howl of anguish from the British Medical Association. The GMC's responsibility for regulating undergraduate medical education would be transferred to another body.

Sir Liam said: "The vast majority of doctors practise medicine of a very high quality. The proposals I have made are designed to help those doctors to maintain their standards and achieve excellence as much as to identify poor practice."

Between 2001 and 2005, 1,700 doctors were referred to the National Clinical Assessment Service because of concerns about their performance, an annual rate of about one in every 100 senior doctors. More than 500 "alert" letters had been issued to the NHS since 1997 warning of dangerous performance, sexual indecency or addiction problems among doctors.

The BMA responded angrily to the proposal to lower the standard of proof for doctors accused of misconduct, saying it was "highly likely to attract huge criticism". James Johnson, chair of the BMA, said: "It opens the door to miscarriages of justice which will devastate the lives of doctors and their families."