Even before the London bombings, the Department for Transport had begun a safety review in light of the Madrid train bombings.

Three million passengers pass through the London tube system every day, while 8,000 buses carry 6.3 million passengers across the capital.

In the short term the police have responded with high profile deterrence, positioning officers near ticket barriers.

In a Commons statement in March, the transport secretary, Alistair Darling, listed proposed improvements including enhanced use of CCTV, an "enhanced national covert explosives test programme", and mandatory security training. But those measures, Mr Darling cautioned, should be "proportionate, pragmatic, sustainable and ... not place an undue burden on industry".

No timetable was given for their implementation and no details provided about the technologies being assessed.

The London Chamber of Commerce and its Asian Association, which represents 1,500 firms, yesterday called for a more radical response.

"High-profile targets should relocate security checks from reception areas to being physically outside their buildings," a joint statement from the organisations suggested. "This move has been effective in thwarting suicide attacks in other parts of the world."

"Devices which detect passengers who are carrying explosives should be installed on the underground."

Greater use should also be made of "stop-and-search powers" on those "carrying large bags or rucksacks".

A new generation of body and baggage scanning systems could be introduced soon, a leading government security adviser suggested yesterday. Simon Stringer, managing director of the security division of Qinetiq, said wave sensors could screen people as they put their tickets through the gates before entering platforms. Passengers could be slowed down by making them walk through an S-shaped barrier.

Qinetiq - which is 56% owned by the government and 30% by the Carlyle group - is developing a "passive millimetric wave" scan which could screen people in real time and without the radiation risks associated with x-rays, Mr Stringer said.

He called for greater emphasis on protective security - trying to prevent an attack before the event.

However, he admitted there was no "silver bullet", no single technology which could solve the problems.

CCTV pictures have already proved invaluable in identifying the suicide bombers as they approached King's Cross station on July 7.

Some interest is being shown in Ipsatac, a video-monitoring system supposed to generate automatic warnings of unusual movements such as overcrowding on platforms. It is being tested in Rome's underground system.

London Underground yesterday acknowledged growing public unease. "If the police and the security services ask us to implement new procedures then we will do so without hesitation," said an LU spokesman.

Some tube drivers refused to work after Thursday's attacks because of the danger, Bob Crow, general secretary of the Rail, Maritime and Transport union, said yesterday.

"There's now a necessity to have a second person on the train," he insisted, after meeting Mr Livingstone.