A new type of bird flu drug which the deadly virus cannot become resistant to is being developed, scientists have revealed.

Governments are currently stockpiling supplies of Tamiflu and Relenza, the best drugs currently available for fighting influenza, in case of the feared world-wide bird flu epidemic.

But because of the way they work, these drugs could quickly become ineffective if the virus mutates and develops resistance.

If a mass outbreak of bird flu strikes doctors could be left with no treatment available. Experts fear avian flu could kill millions if no vaccine or anti-viral drugs can stop it.

Effective vaccines are difficult to produce in advance because the virus is constantly changing, so doctors will rely on anti-viral drugs which act to slow down the rate the virus spreads through the body.

But tests have already shown that bird flu can become resistant to anti-viral drugs at alarming speed.

However, a new type of drug is being developed which would not suffer from the same problems as Tamiflu and Relenza because it works in a different way.

All three drugs act on the same target, a protein enzyme called neuraminidase on the surface of the football-shaped flu virus. This enzyme is vital in how the virus spreads.

Tamiflu and Relenza, types of neuraminidase inhibitors, work because they resemble the molecules that the enzyme is meant to work on. When a molecule of the drug comes into contact with the enzyme it locks into place and stops it from doing its job.

But these two drugs are inherently susceptible to resistance problems as the virus can mutate and change the shape of parts of its surface, meaning the drug can no longer bind to it. The problem has been highlighted by recent research.

In a study of eight bird flu patients in Vietnam being treated with the drug, two showed signs of recovering before dying, suggesting the virus had been able to mutate rapidly.

In another trial of Japanese children being treated with Tamiflu for ordinary human flu, the virus developed resistance in one-in-six patients.

The new type of molecule being created also acts on neuraminidase, but rather than physically blocking the enzyme, it undergoes a chemical reaction with one specific part of the protein. This completely stops the virus from reproducing itself and means it cannot develop resistance.

Dr Andrew Watts from the University of Bath and Dr Jennifer McKimm-Breschkin from CSIRO in Melbourne, Australia, have just been awarded a research grant of £408,000 from the Medical Research Council to develop the new class of inhibitor they have discovered into a new kind of anti-viral flu drug.

They hope to have a treatment ready for clinical trials within three years.

Dr Watts said: “Tamiflu and Relenza remain our best lines of defence against a flu pandemic, but we need to be working on better alternatives that do not suffer the same inherent resistance problems.

“We are still in the very early stages of the development of this class of molecules into a drug, but the initial tests, and what we already know about how it works, are very, very encouraging.

“By the time the influenza virus becomes fully resistant to the drugs currently in use, we should have this as a viable alternative.”