The inquiry follows two break-ins yesterday at public houses by three masked men. In the second raid a barmaid was shot in the leg.
Scotland Yard are investigating possible links between at least five more similar attacks on pubs around the north-west section of the M25 this summer.
In the latest incident Melanie Nurden, 20, was hurt after the gunmen forced a first-floor window at The Bell in London Colney, Hertfordshire. The burglars held Miss Nurden and Karen Cowley, the licensee, and her two daughters at gunpoint and demanded money.
They were given pounds 150 but as they asked for more, they shot Miss Nurden in the leg before fleeing. She was last night described as "stable" in hospital.
An hour before the raid on The Bell, three men in balaclavas forced a licensee to open his safe at gunpoint at a pub 20 miles away at Takeley, Essex. Police are also investigating a raid on a pub in Barnet, north London, nine days ago.