It was the first seminar on transport safety since the 7 July bombings

Speakers at the seminar on making the transport system safe, the first since the bombings on 7 July, included police chiefs and a former IRA member.

Met Police commander Roy Ramm said leaving bags was not just a nuisance but could "cause panic and divert valuable policing resources."

"People need to understand that it just isn't something you can do any more."

He said it should be made a fixed penalty offence - similar to fare-dodging or smoking.

London Underground has put up signs warning people not to leave bags unattended as security scares have been causing further delays since the bomb attacks on three Tube trains and a bus on 7 July.

More bus conductors

Other suggestions at the seminar, hosted by think tank Policy Exchange, included putting conductors back on the buses.

New York Times writer Jonathan Foreman said national guardsmen on the city's transport systems after the 11 September attacks on New York had reassured the public and reduced crime.

Former IRA man Sean O'Callaghan said the lack of bus bombings by the IRA during the Troubles was in part due to the fact that conductors could spot abandoned bags or suspicious behaviour.

In 2003, London mayor Ken Livingstone scrapped plans to put a conductor on every bus, because he said the time they would save collecting tickets was not worth the �350m annual cost.

Other recommendations included strict vetting of Tube staff and more uniformed police in stations, on platforms and on trains.