The news that the flying squad is to have its numbers cut because of the decline in armed robberies in the capital prompts the question: where did all the bank robbers go?

The golden age for armed robbery – golden for the robbers rather than the petrified cashiers – was in the 60s and 70s in an age before every street corner had a lurking CCTV camera, and before supergrasses tore up the old "honour among thieves" code of conduct book. All that the wide boys of London needed then was a couple of sawn-off shotguns, a few stolen cars, a bunch of balaclavas, a local bank and the urge.

Bank robbers became the criminal aristocracy, the "faces" who always had cash to splash. Even in the 90s, bank robberies were an almost daily occurrence in London – there were 291 in 1992 – but now they seem almost as old-fashioned as an edition of The Sweeney, the TV series that celebrated the flying squad and flared trousers. There were only 26 such robberies in the UK capital in 2012 and the figures nationally show the same decline, from 847 down to 108 in the 20 years from 1992.

So what happened? Did bank robbers find God or take mindfulness courses? Jackie Malton, herself a former flying squad officer and the model for Prime Suspect's Jane Tennison, reckons there are a number of factors to the decline. "It's just too risky nowadays," says Malton, who now works with addicts in prison and as a TV programme consultant. "Because of all the increased security they have a much higher chance of being caught and a heavy sentence – 16 or 17 years – is a reality. Crime is going online now."

A ‘bank job' explained: an annotated photograph from 1971 showing how robbers tunnelled into a branch of Lloyd's in London. Photograph: Wesley/Getty

Certainly, a prime reason for the decline is the increasing sophistication of security, from those CCTV cameras to dyed banknotes. And back in the 70s, robbers didn't have to worry about a stray hair on an abandoned balaclava leading back to them via their DNA.

Another reason for the decline is the closure on cost grounds of many local branches so that banks can carry out their vital function of protecting the bonuses of their senior executives. Since 1990, more than 8,000 banks – that's more than half of the high street branches – have closed. My two former local branches have both closed, one to become a bookies and the other a wine bar. In many banks now only the luckiest robber would be able to squeeze past the long queues and the harassed staff long enough to blurt out his (it always was a boy's game) demand for cash.

Another factor is that the old criminal eleventh commandment of "thou shalt not grass" collapsed in the 70s, with the arrival of the supergrasses, who parlayed their way out of long sentences by giving evidence against their former colleagues. As almost all such robberies needed a team of four or five, there was an increasing chance that one of them would go "talk and walk" on them.

At the same time, many of those armed robbers, during their long stays in prison, met up with the new breed of drug smugglers and realised that there was a simpler way to make large amounts of money that involved fewer risks and, if caught, shorter sentences. And now, of course, the smart criminal goes online. Who needs a sawn-off when a laptop lets you into the estimated £27bn worth of online fraud carried out annually?

Whatever failings there may in the Metropolitan police at the moment, there is nothing approaching the institutional corruption that existed among detectives in some parts of the capital in the 70s. The late bank robber Bobby King, who carried out dozens of heists in London, used to say that it was always a relief when the police came calling after a robbery, asking to be paid off. "I can't remember one local CID who wasn't crooked," he used to say.

And some of the chaps who might once have thought of armed robbery have wised up and seen they can get hold of lots of loot in other, less risky, ways. As Bertolt Brecht said: "What is robbing a bank compared to founding one?"