Ministry of Defence scientists have taken samples from the British survivors of the Moscow theatre siege in an effort to identify the mystery gas used in the storming of the building.

The move comes as Foreign Secretary Jack Straw says he will put pressure on Russia to say why it will give no information about the gas.

Blood samples and clothing swatches have been taken from Sidica Low and son Richard, from Southgate, north London, for testing at the MoD's Porton Down laboratory.

Doctors need the formula of the gas

The Russian authorities have refused to reveal the nature of the gas that is believed to have killed more than 100 hostages.

Current speculation centres on either an anaesthetic gas called halothane or some form of opiate-laden gas, perhaps based on fentanyl.

An MoD spokeswoman told BBC News Online: "The tests are not for the purpose of treating or advising. Porton Down has taken an interest in the substance and has taken samples."

Critical condition

Porton Down is one of the foremost centres in the world for research into chemical weapons, with many of the scientists there having considerable experience from the Cold War years.

But it is not clear who the MoD would inform should they discover the nature of the gas.

With many of the hostages still in a critical condition in hospital because of the gas, doctors say their treatment would improve if they knew what gas they were dealing with.

Mr Straw told BBC Radio 4's Today programme: "Yes we will be discussing these matters in private with the Russian authorities and for certain we will be seeking an explanation from them as to why they believe it is necessary in their national interests to keep this information secret."

But he added: "But am I going to criticise, no, because [the Russian authorities] were posed with the most difficult choices."