The government has admitted Britain faces a major shortage of a painkilling drug vital for cancer patients and accident victims.

The department of health said diamorphine, the medical version of heroin and one of the most powerful painkillers known, could run out within weeks.

Professor Mike Richards, national cancer director, said: "We have taken immediate action and are working hard to make sure that this potential shortfall does not, as far as possible, affect the care and quality of life of patients.

"Patients should be reassured that we will do all we can to minimise the effect this disruption to supply may have."

Richards said the drug company Chiron had said there would be problems providing diamorphine and that supplies were unlikely to improve before March.

Chiron is already in difficulties because of the closure of its flu vaccine plant on Merseyside, which has disrupted supplies of the vaccine here and in the US.

The Conservatives reacted angrily to the news.

Simon Burns, health spokesman, said: "Many cancer patients will suffer needless pain due to the disorganisation of this shambolic government.

"Blair’s government must bear the responsibility if any patient has to suffer additional pain because of the shortage. This is a fiasco that the government could have prevented."