Patients may be forced to undergo extra unnecessary tests because of a shake-up in doctor regulation, medics say.
Doctors are facing a shake-up in regulation
Delegates at the British Medical Association conference heard how doctors feel they needed "evidence" to safeguard against potential claims.
Plans published in February in the wake of the Shipman murders said doctors face being struck off on less proof.
They are also due to lose the right to police themselves under government reform of the General Medical Council.
The white paper said the GMC would lose the right to adjudicate in fitness to practise cases when complaints were made against doctors.
It would still investigate the complaints, but the final decision on what happens to a doctor would most likely pass to an independent tribunal.
The civil standard of proof - "on the balance of probability" - along a sliding scale depending on the seriousness of the allegations would also be used in hearings.
This would replace the current criminal standard of "beyond reasonable doubt".
Harold Shipman, from Hyde in Greater Manchester, is thought to have murdered more than 200 people over a period of more than 20 years.
In her 2004 report, inquiry chairman Dame Janet Smith said the GMC was more focused on the interests of doctors than patients.
But at the meeting in Torquay said the plans were a politically-motivated attack on the profession and would lead to defensive medicine.
Dr Laurence Buckman, deputy chairman of the BMA's GPs committee, told the conference the key issue was about the sliding scale of proof.
"If standards of proof fall, most doctors will practise with an eye on making sure that the balance of evidence on what they are doing falls in a safe zone.
"Doctors will make sure they practise with a bigger safety margin and with a eye to the risks to themselves.
"Patients will need more investigations and it will lead to a different way of treating them that's more risk averse."
He said patients would be put through procedures so doctors felt protected if a case was made against them.
He also said the GMC was at risk of becoming a "government poodle".
Doctors voted overwhelmingly in favour of a motion saying self-regulation was "essential" for the medical profession and that the standard of proof in fitness to practise hearings must remain at "beyond reasonable doubt".
But Barbara Wood, who sits on the BMA council and heads up its patient liaison group, said the white paper reflected "real patient concerns".
"When some doctors do fall below the standards required to provide safe and proper care, some are not recognised and their patients remain at risk."
But a spokeswoman for the Department of Health said the reforms had been designed to speed up the regulation process.
She added: "The sliding scale of proof will mean the most serious cases still require the criminal standard so doctors should not have to alter their practises."
Dr Hamish Meldrum, chairman of the BMA's GPs committee, has been appointed as the union's new chairman.
He replaces James Johnson who resigned last month.
He was elected by the BMA Council in a three-way contest with deputy chairman Dr Sam Everington, who had been acting chairman since Mr Johnson's resignation, and Dr Tony Calland, chairman of the BMA's ethics committee.