The head of a private drug addiction centre with an international reputation has been found guilty of inappropriate and irresponsible prescribing by the General Medical Council.

After the longest and most complex case in the council's 145-year history, Colin Brewer, 64, the medical director of the exclusive Stapleford Centre, which runs clinics in Belgravia and Stapleford Tawney, Essex, and two of his medical colleagues, Hugh Kindness, 66, and Ronald Tovey, 47, were found to have handed out large quantities of addictive drugs to patients without assessing their needs or monitoring their progress.

The GMC's disciplinary panel found that their actions put their patients' health at risk and increased the likelihood that the drugs would be diverted to the black market.

All three could be struck off the medical register by the GMC if their actions are considered to have fallen below the standard expected and to have brought the profession into disrepute. Four other doctors at the centre were cleared - Anthony Haines, 75, Nicolette Mervitz, 30, Martin O'Rawe, 46, and Timothy Willocks, 47.

The seven doctors were charged with serious professional misconduct in a case that started in October 2004 and has taken 114 days of hearings and cost millions of pounds.

Dr Brewer's supporters claimed that it was a test case which would determine how drug addiction was treated in Britain but his critics said it was a straightforward matter of whether he and his colleagues had given their patients proper care.

In one case, Dr Brewer provided a home detoxification kit to a patient, Grant Smith, 29, which contained 16 different drugs. Mr Smith choked to death on his own vomit after taking the drugs.

The GMC's disciplinary panel found that the instructions for the kit were "complex, unclear, confusing and inadequate" and concluded that it was "not a safe or suitable method of managing a patient who was being treated with large doses of potentially dangerous drugs and supervised by untrained carers".

Of 12 cases considered by the GMC, the panel found that Dr Brewer's care had been inappropriate and irresponsible in 11 of them.

On many occasions he issued single prescriptions for more than 1,000 tablets of different drugs, for which there was a high demand on the black market. In the case of one patient, who was unemployed and worked intermittently as a labourer, his prescriptions were collected on 34 different occasions over three years by six different people, some of whom were known addicts.

Some of his patients were unemployed or worked in low-paid jobs but no inquiries were made as to how they could afford the clinic fees of £100 to £200 a week.

Dr Brewer founded the Stapleford Centre in 1987, at the request of the Home Office, to treat addicts who had formerly been looked after by a private Harley Street practitioner. He claimed to provide addicts with maintenance doses of drugs, sometimes over many years, which enabled them to hold steady jobs and lead stable lives, as well as offering detoxification treatment to help wean them off drugs.

But he found himself on the wrong side of the Home Office as he pursued a liberal prescribing policy, arguing that addicts could not be forced to give up their drugs until they were ready.

It was better to give patients the drugs they needed and keep them in touch with medical services than abandon them to the black market, he said.

His critics from the NHS drug treatment service argued that the Stapleford Centre was acting like a drug "grocery", giving addicts the drugs they wanted in quantities so large that they were fuelling the black market.

The GMC will consider next week whether the facts proved against Dr Brewer and the other defendants amount to serious professional misconduct and what penalty, if any, to impose.