Police to snoop on chatting.
CCTV cameras fitted with high-powered microphones could eavesdrop from up to 100 yards away.
Officials from the Met, City of London police and Transport for London are investigating the technology and may use it during the Olympics.
More talks with VCS Observation, the Dutch firm behind the equipment, are due next year.
It could cause a storm from civil liberties groups.
Graeme Gerrard, of the chief police officers' video and CCTV group, admitted there would be "legal obstacles" to using the microphones.
He added: "We need a public debate."
In Holland, 300 "listening cameras" have been fitted in Groningen, Utrecht and Rotterdam.