Britain is increasingly concerned there is evidence that chemical weapons have been used in Syria.
Foreign Minister William Hague said the claims must be urgently investigated and perpetrators held to account.
British intelligence sources have told the BBC that soil samples gathered from random sites in Syria, have been smuggled to Britain for testing.
The evidence suggests "some use of chemical weapons", but it is not clear by which side.
Syria is believed to have stockpiles of mustard gas and sarin, a highly toxic nerve agent.
A team of UN experts is in Cyprus awaiting permission to enter Syria to investigate a chemical incident last month in Khan al-Assal, Aleppo province.
Mr Hague called on the Syrian regime to co-operate fully and allow the UN team unfettered access.
''They should take heed that the world is watching and those who order the use of chemical weapons or participate in their use must be held to account," he said in the House of Commons.
