The BBC's Andrew Hosken reports on what exactly is going on

The vaccine has been developed in response to fears that British forces could be attacked by countries, such as Iraq, which are thought to have built up stockpiles of biological weapons.

Defence experts have long suspected that the Iraqi leader, President Saddam Hussein, has developed biological weapons to spread diseases such as anthrax and possibly bubonic plague.

The research programme is being carried out on behalf of the Defence Evaluation and Research Agency (Dera), the body which runs the government's top-secret Porton Down chemical and biological warfare centre in Wiltshire.

Porton Down's technical director, Dr Rick Hall, told BBC Radio 4's Today programme: "We have a new vaccine against plague that is at a particularly advanced stage of development, that is produced by genetic engineering.

"And of course once it is licensed - through exactly the same process as any other medicine - it will not only provide increased protection for the UK and its armed forces, but it will also be available to help protect civilians in parts of the world where plague occurs naturally."

Civilian and military benefits

According to a report in Scotland's Herald newspaper, the existence of the programme emerged as a result of a Parliamentary question by Labour MP Joan Ruddock.

The newspaper says Dera chief executive John Chisholm wrote to Ms Ruddock confirming the research was taking place.

He said "genetic-modification techniques" were a scientific research tool used at Porton Down "to develop protective measures for the UK and its armed forces in the event of exposure to biological weapons".

Tests of the GM vaccine for the plague involving human volunteers are to start imminently. The vaccine could have civilian as well as military benefits.

A BBC correspondent says research into genetically-modified bacteria, including the bubonic plague, is one of Britain's closest-guarded military secrets.

He says often the organisms are modified to make them less dangerous and provide protection to those involved in the experiments.

'Reasonable precaution'

Menzies Campbell, defence spokesman for the Liberal Democrats, said he would have been "disappointed and surprised" if Porton Down had not been carrying out experiments for defensive purposes.

Liberal Democrat Defence Spokesman Menzies Campbell: "We've still got a long way to go"

He told BBC Radio 4's Today programme: "There are regimes which are willing to use chemical weapons, and it seems to me to justify the view that we should take every reasonable precaution we can, including scientific research to prevent British forces and civilians from being at risk."

But he said the whole testing process should be "more open".

He also spoke of the dangers of "marrying together" chemical weapons and missile technology, "where it's easy to fit a biological warhead to a missile with a range of thousands of miles".

'UK plague death'

An environmental group opposed to genetic engineering, Gene Watch, says genetically modifying bacteria is dangerous because the results can be unpredictable.

Porton Down is currently at the centre of an investigation by Wiltshire detectives over allegations that a serviceman died in 1953, after taking part in a Sarin gas experiment.

It is claimed he thought he was taking part in a programme designed to find a cure for the common cold.

According to Rob Evans, a journalist who is researching a book into the experiments, one of the research centre's own scientists, Geoffrey Bacon, died of the plague in 1962.

Bubonic plague is as an acute infectious fever characterised by chills, prostration, delirium and swollen glands in the armpit or groin.