POLICE are hunting for someone who threw a concrete block from a Northwood road bridge on to a tube train passing underneath.

The block, about the size of a brick, dented the roof of a Metropolitan line carriage and forced London Underground to suspended services while the debris was cleared.

British Transport Police (BTP) officers attended Northwood Station late Wednesday afternoon after receiving a report of the incident from rail staff at 5.19pm.

The block was dropped from Highfield Road, about 400 metres south of Northwood Station. No one was injured by it but police described it as "incredibly irresponsible".

A BTP spokesman said: "This was an extremely foolish and dangerous act and it was lucky no one was injured. Officers immediately searched the area but to no avail.

"Throwing stones at trains is an incredibly irresponsible thing to do and we urge those who think that throwing stones at trains is fun to think about the consequences of their actions, which in this case could have been much worse."

Anyone with any information about this incident is asked to call 0800 40 50 40 quoting incident number 491 of 29/2/2012. Alternatively, call Crimestoppers on 0800 555 111.