Health experts are warning that a new bug which is resistant to antibiotics has been brought into the UK by patients treated abroad.
The new strains of bacteria, called Enterobacteriaceae, have been identified in UK hospital patients, "a significant proportion of whom had received medical treatment abroad", according to the Health Protection Agency (HPA).
The HPA said that one strain - called NDM-1 (New Delhi Metallo-1) - is "swiftly emerging".
A total of 22 bacteria with the NDM-1 enzyme have been identified from 19 patients at 17 hospitals.
There has been one instance of possible transmission between two patients.
"Critically, at least nine out of 19 affected patients have had recent hospitalisation in India or Pakistan," the HPA said.
One patient received blood treatment in India while two had undergone cosmetic surgery in India.
Others had received renal or liver transplantation in Pakistan.
The HPA said NDM-1 had been repeatedly imported to the UK from the Indian subcontinent, "though there may now also be UK circulation since some affected patients have no immediately identifiable overseas links."
In a statement, it added: "The HPA has been warning for some time of the risk from infections which are more difficult to treat due to antibiotic resistance.
"The recent emergence of a new group of enzymes (carbapenemases) carried by some bacteria is of concern because these enzymes prevent carbapenem antibiotics from working effectively, making the bacteria resistant to treatment.
"This re-emphasises just how important it is that clinicians in hospitals are vigilant in monitoring the emergence of multi-resistant bacteria.
"Identifying possible resistance among infections is particularly important among patients who have been in hospitals in the Indian subcontinent and the Eastern Mediterranean including Greece, Turkey and Israel, where carbapenemases have been identified.
"Carbapenams are powerful antibiotics that are used to fight gram negative bacteria, particularly the multi-resistant ones.
"Other treatment options are available to fight infections caused by these carbapenemase-producing bacteria but they present major challenges for clinicians and will often demand combinations of antibiotics to be used.
"At present this type of antibiotic resistance is comparatively rare in the UK, with testing during 2008 and so far in 2009 having found 20 cases. However, careful monitoring of the situation is needed.
"Antibiotics are a precious resource in fighting infections and one that we must do everything possible to preserve.
"Multi-resistant gram-negative bacteria pose a notable public health risk and it remains important that the pharmaceutical industry continues to work towards developing new treatment options, in the same way they have done for gram-positive bacteria like MRSA."
Professor Brian Duerdan, the Government's inspector of microbiology and infection control, said: "We have been aware of this issue for several months and, after advice from the HPA, we alerted infection specialists in the NHS to the possibility of importation and potential spread of these bacteria.
"This will help us monitor the situation and respond appropriately.
"Maintaining proper infection control is one of our priorities and this, together with prudent antibiotic prescribing, will help us control these carbapenemase-producing bacteria.
"However, to help the NHS, we have commissioned authoritative professional guidance to identify whether further control measures need to be applied for this type of bacteria."