Stocks of a powerful painkiller used to treat thousands of NHS patients may run out in the next few weeks, the government has warned.
Diamorphine will be rationed
Suspension of production by the main supplier of diamorphine, Chiron, means stocks could fall to critical levels in the next few weeks.
Doctors are being asked to ration the drug to those most in need, such as patients with advanced cancer.
The government said the shortage was unforeseeable and it was acting fast.
The other UK manufacturer of injectible diamorphine, Wockhardt UK, has been contacted for extra supplies.
The Department of Health is also sourcing extra stocks of alternative painkillers, such as morphine.
It is unlikely that further supplies will be available from Chiron before the end of March.
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, impact on the care and quality of life of patients.
"It is a serious problem, but it is one that can be handled.
"Patients and families should be reassured that we will do all we can to minimise the effect this disruption to supply may have."
About 20,000 ampoules of diamorphine are used in the NHS each day.
Shadow Health Minister Simon Burns 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."