A spate of more than 30 raids in and around London, in which groups of youths aged between 13 and 15 have rushed behind the counters to grab money from tills, staff and customers before escaping, has led to the formation of a Scotland Yard unit to combat the problem.
Building societies and post offices have also been hit, and in many branches new security equipment is being specially installed.
The police first became aware of steaming - an idea imported from the United States - when gangs of youths raced through London underground trains stealing from passengers. Later it spread to buses and crowds.
Its use against high-street money outlets follows a gradual decline in the number of armed robberies against banks and building societies, which has been largely due to the growth in sophisticated security devices such as closed circuit television.
The success of steaming depends on surprise. In most cases the "steamers" wait until a member of staff opens a security door and then suddenly push through into the private area behind the tills. They also jump over the front counters and snatch money from customers queuing.
The steaming gangs started to target banks towards the end of last year: there were 14 cases in 1995, most of which were in London. So far this year there have been about six such bank raids, and there have been about five incidents of steaming at building societies in the same period.
The societies have set up a special warning system to alert their members if any suspicious groups of youths are spotted, while banks have for the first time added a special category of "steaming" raids to their security records.
"It's something we have become concerned about," said Brian Capon, spokesman for the British Bankers' Association. "Gangs of up to 11 people are taking cash from the banks. In some cases they steam their way into the secured area - as a member of staff goes through they are pushed aside."
"The steaming gangs are often in groups of about six, and in their early teens," said Louise Coffey, under-secretary of personnel at the Building Societies Association. "Because they are small and quick they can vault over the counters or they wait by the doors the run through.
"If any suspicious gangs are seen in or around the building societies, faxes are sent to all nearby branches to warn them. It's like going on red alert."
Tony Wells, spokesman for the Abbey National, said: "Steaming raids are on the increase and as a result we have made several changes to our security arrangements. Some smaller societies have recognised the need for better security." A reward of pounds 15,000 is being offered to anyone who helps a building society prosecute someone involved in robbery.
Security measures such as automatic rising screens and mirrors over doors have been fitted, although all the banks and building societies contacted refused to discuss specific measures.
The Metropolitan Police have set up a team in Clapham, south London, to investigate and monitor the phenomenon across the force area.Reuse content