A BBC investigative journalist on the Inside Out programme 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 Agency (UKBA) 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 is 20 times more powerful than the 50,000 volt weapons carried by police.
Eran Bauer, of Civil Defence Supply - which supplies Tasers to the UK police - 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."
Both stun guns came with a free gift: a can of pepper spray
Reformed gang member Daryl 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.
Polie executives declined comment, but former murder squad Detective Chief Inspector Pete Kirkham says stun guns have become weapon of choice for many criminals.
The UK Border Agency said it was working with Revenue and Customs and police to target importation of illegal weapons.
The BBC says that it will hand the stun guns to police.