The General Medical Council, doctors' regulatory body, found Dr Archibald Alexander guilty of serious professional misconduct.

Dr Alexander was accused of prescribing lethal doses of methadone, the heroin substitute, to two inmates at Brixton Prison in 1994.

Carl Owens and David Davies, both aged 22, died of fatal overdoses in May and September 1994 respectively.

Owens had a heart attack after taking a methadone overdose. He fell into a coma and died.

Davies died of an overdose on the day he was due to be released after serving a seven-day sentence for non-payment of an �85 fine.

Dr Alexander gave both Owens and Davies 50ml of methadone and a twice-daily dose of valium.

Dr Alexander, who is retired and suffers from Parkison's Disease, was accused of not checking the medical records of the inmates thoroughly enough.

Both Owens and Davies reportedly told him they had a history of drug use in order to get methadone to relieve the gloom of prison life.

But neither was an addict and the methadone dose they received was too strong for them.

Another doctor told the GMC that he had had doubts that they were addicts. Suicide

Dr Alexander, who did not attend most of the hearing due to his illness, was cleared of a charge related to the suicide of 29-year-old Kirk Chean.

Dr Jeremy Lee-Potter of the GMC said all patients - including those in prison - were entitled to good standards of practice and care.

He added: "A conscientious assessment of the history, symptoms and signs of a patient's condition is a fundamental aspect of good clinical practice and is essential before initiating treatment or prescribing drugs."

He said Dr Alexander's actions were "irresponsible", even though he had been misled by the two inmates.