Doctors' leaders have urged MPs not to vote for a private members bill tightening anti-euthanasia laws.

This is despite their tough stance against any sort of "mercy killing" by doctors.

The British Medical Association (BMA) believes that the bill, due for its second reading in the House of Commons on Friday, is draconian.

It says it will mean that doctors will be forced to carry on giving treatment to patients - even if the patient wants treatment to stop.

The bill would make it an offence for any doctor to intentionally hasten the death of a patient.

This, maintains the BMA, means that patients would not be able to refuse further treatment.

In a letter to all MPs, BMA chairman Dr Ian Bogle said: "Not only would contemporaneous decisions be cast into doubt, but attempts by patients to make an informed advance refusal would also by significantly hampered.

"The Bill would confuse doctors about what they could do in making decisions about treatment."

He gave an example scenario in which a patient undergoing dialysis for chronic kidney disease developed a terminal cancer.

Although the patient might, in the final stages, wish to stop the dialysis treatment in order to be more comfortable, under the Bill a doctor might not be permitted to agree to stopping the treatment, as it would arguably be hastening the patient's death.

Mrs Winterton introduced the Bill after the BMA published guidance on the withdrawal of medical treatment from patients.

Every such decision in the UK still has to be tested in the High Court, although some groups fear that this is not being done in each case.

After winning first place in the ballot for private members bills in December, Mrs Winterton said: "My Bill will make it abundantly clear to doctors that they cannot intentionally bring about the death of their patients either by action or by omission.

"Many people, particularly the disabled and elderly are increasingly fearful of entering hospitals because the BMA - without legal authority - has issued guidelines to doctors allowing withdrawal of medical treatmentand of tube feeding from patients who are not dying."

However, it is difficult to prove "intent" on the part of a doctor that treatment - or lack of it - was intended to hasten death.

A recent criminal case involving Dr Dave Moor, a Newcastle GP, who gave a large dose of morphine to a terminally-ill patient, heard evidence that the morphine was intended to relieve pain, and Dr Moor was acquitted.