The mayor said they cannot install airport-style security

Ken Livingstone told MPs it was one option being considered to make the network safer after July's bombings.

An all-party Home Affairs Select Committee heard the mayor say he is looking to see if a "sort of sniffer technology" could be developed.

But Mr Livingstone added the airflow caused by Tube trains made it difficult to set up such as system.

Mr Livingstone also said it would prove very difficult to install airport-style security on the Underground.

"For every person who goes through Heathrow, 15 will go through the Tube," he said.

Repeating mistakes

"There isn't physically the space for the equipment.

"It would also mean huge tailbacks and would mean a dramatic change in the time people take for a journey."

He revealed that there would be 100% closed circuit television coverage on London's buses by the end of the year on both upper and lower decks, compared with the present 95% coverage.

He also hopes over the next five years to double the number of CCTV cameras on the Underground and improve technology so that the images get better.

He told the committee Muslim extremism could be tackled without repeating the mistakes made in the 1970s when addressing IRA terrorism.

"We have to ensure that we don't make the same mistakes," he said.

The attack on London's public transport system on 7 July claimed 52 lives.