A German carpenter has invented a valve which he claims will revolutionise contraception, by allowing a man to turn the flow of sperm from his testicles on and off at the flick of a switch.

Clemens Bimek said the idea first came to him 20 years ago, when he was watching a television documentary about contraception. When he discovered no one had ever filed a patent for such a device, he developed his own.

"Many of the doctors I consulted didn't take me seriously. But there were some who encouraged me to go on tinkering and helped me with their expertise," Mr Bimek said.

Now his valve is to be implanted in 25 men in trials starting this year.

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It is nearly 2.5cm long and weighs less than 2.8g. It is surgically implanted on the vas deferens, the tube that carries sperm from the testicles, in a half-hour operation, and controlled by a switch beneath the skin of the scrotum.

So far Mr Bimek is the only man who has the switches implanted, one for each testicle. He underwent the operation under local anaesthetic so he could help direct the surgeon.

Hartwig Bauer, the urologist who carried out the surgery, told Spiegel magazine that the valve was preferable to a vasectomy. "A third of patients want to have the [vasectomy] operation reversed later, but it doesn't always work," he said.

But other doctors have expressed concerns over the new technology.

"Implanting the valve could cause scarring where it meets the vas deferens," said Wolfgang Buehmann, spokesman for the Professional Association of German Urologists. That could prevent sperm flowing even when the valve is open. He also warned the valve could become clogged.