Stun guns with a voltage 20 times stronger than police Tasers can be illegally imported into the UK with ease, a BBC investigation has found.
Possessing a stun gun carries a maximum jail sentence of 10 years but Inside Out bought two devices by mail order - which both made it through customs.
The BBC also obtained figures showing police have seized hundreds on the streets of London in recent years.
The UK Border Force (UKBF) insists it is tackling the problem.
Statistics obtained under the Freedom of Information Act show the Metropolitan Police has investigated more than 200 crimes involving the weapons in the past three years.
Over the same period they seized about 500 stun guns.
The weapons are legally manufactured abroad - but cannot be bought in the UK.
Yet there were no questions about age or criminal status as the BBC ordered a pair of powerful stun guns from a German website for £60 each.
Both weapons, sent separately, arrived in days. And each came with a free - and illegal - pepper spray.
At one million volts, one of the guns had a voltage 20 times that of the 50,000 volt weapons carried by police.
Eran Bauer, of Civil Defence Supply - which trains the UK police in using Tasers - described it as "one heck of a brute".
After seeing it fired he said: "That's lightning - horrendous tool. I don't believe you managed to get one of these. That is seriously dangerous."
Mr Bauer explained: "You are going to end up with skin burns for a start, and it's subcutaneous damage.
"If it goes into the neck across the nerves you can just imagine the damage.
"Hit across the chest then anything could happen - [it could] end up killing someone."
He added: "This has got to be stopped at source. There must be some way of screening mail."
Reformed gang member Daryl James believes the growing appeal of stun guns for teenage gangs is a result of the crackdown on knife and gun crime.
'People find alternatives'
He said: "You can't have all these youngers [young gang members] running about with stun guns - they'll stun everybody.
"As we move forward into knife bans, gun bans, people find alternatives."
Mr James said stun guns were a more effective tool for bank robbers than guns or knives - as they left victims momentarily paralysed with less risk of death.
The Met declined to be interviewed on the issue.
But former murder squad Detective Chief Inspector Pete Kirkham believes stun guns have become the weapon of choice for many criminals.
He said: "Stun guns have been used in armed robberies.
"They turn up in aggravated burglaries, in disputes between drug dealers - that's the range of serious crimes."
Mr Kirkham said front-line police were exasperated by the ease with which stun guns made it past customs.
The UKBF was unable to say how many stun guns it had seized last year.
A spokesman said the most recent year for which it was able to provide figures was 2008/2009 - almost four years ago, when 160 were seized.
Labour has called it a "worrying admission", adding: "If UKBF don't even know how many stun guns have been stopped at the border, how can it possibly address this problem?"
Keith Vaz MP, chairman of the Home Affairs Select Committee, said: "I'm absolutely shocked there are present on the streets stun guns more powerful than weapons being used by our police.
'Must take action'
"How is it possible for someone to go online and get a gun into this country through what we are told is one of the most sophisticated borders in the world?
"Clearly it's important we take action to find out what's gone wrong and I intend to raise the matter with the head of the Border Agency and the home secretary when she appears before the committee."
The UKBF declined to be interviewed but in a statement it said: "The UK Border Force, Revenue and Customs and police work together to target importation of illegal weapons, and those who seek to bring weapons into the country."
The agency denied it had stopped counting stun gun seizures altogether. A spokesman claimed the statistics from 2009/2010 on were "in the verification process".
The BBC will hand the stun guns to police.
BBC Inside Out is on BBC One in the London region on Monday, 7 January at 19:30 GMT and nationwide on the iPlayer for seven days following transmission.