British scientists are developing a vaccine to give lifelong protection against all strains of flu.

It would deal with everything from a winter virus to a bird flu outbreak.

Current flu jabs are out of date within a year because the virus mutates so often.

The new FLU-v vaccine is also easier to make than traditional jabs, so it could be stockpiled against a global pandemic.

Flu kills up to 12,000 Britons a year, many of them elderly. A pandemic of the human form of bird flu - which many experts say is inevitable - could claim 700,000 lives in the UK alone.

It is hoped the first human trials of FLU-v will start next year. If they go well, it could be on the market in three to five years.

Dr Stuart Robinson of the Buckinghamshire company PepTcell, which developed the vaccine, said: "We expect one course of injections - probably of two a week apart - to give life-long immunity".

Existing flu jabs focus on a pair of proteins on the surface of the virus. These constantly mutate, however, making it impossible to prepare in advance for each new strain.

The new vaccine is based on other proteins, common to all strains of the virus, which have not mutated for 60 years.

Rather than using antibodies to kill the virus, the new jab relies on the power of other immune system cells called cytotoxic T cells.

Lab tests have shown that FLUvcan save mice given a dose of flu that would normally kill them. Results being presented at a Toronto conference today show that more than half the treated mice survived. Unvaccinated creatures died.

The mice had been genetically altered to give them human-like immune systems - encouraging hopes the jab will work on people.

FLU-v can also be produced quickly and in bulk. Conventional jabs are grown in hen's eggs - a slow process that yields only one shot per egg.

A universal flu jab is the 'holy grail' of researchers, potentially worth a fortune. At least two other companies - British-based Acambis and the Swiss firm Cytos - are also working on vaccines.

Expert reaction was mixed last night. Professor Hugh Pennington, emeritus professor at Aberdeen University, said: "We are in real need of new types of vaccine. Anything that gives a more rapid response would be of great interest."

But virologist Professor Ian Jones, of the University of Reading, warned that an HIV vaccine based on similar principles had failed. He said T cells mop up virus-infected cells, but questioned whether they could guard against infection.

"We all have flu T cells from previous infections," said Professor Jones, "yet we still get seasonal flu."

Professor Malcolm McCrae of Warwick University said the vaccine could wear off more quickly than a normal jab, but might be a useful short-term measure against a totally new strain.