About 100 military personnel have been deployed to Salisbury to help in the investigation into the attempted murder of an ex-Russian spy and his daughter.
They will include Royal Marines and military personnel who have specialist training in chemical warfare and decontamination, the BBC understands.
Sergei Skripal and his daughter Yulia collapsed on Sunday afternoon after being exposed to a nerve agent.
Home Secretary Amber Rudd has described the attack as "outrageous".
The BBC's Daniel Sandford said the military personnel were "experts in chemical, biological, radiological and nuclear warfare".
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The Metropolitan Police said the counter-terrorism unit has requested the military's assistance "to remove a number of vehicles and objects from the scene in Salisbury".