British scientists have made a breakthrough in the search for a male contraceptive pill which could transform the sex lives of millions of couples.
For decades, researchers have tried to produce a reliable alternative to condoms or a vasectomy, but there has been little progress.
Now UK scientists have found the secret of making men temporarily infertile by switching off sperms ability to swim.
They have made tiny designer compounds which smuggle themselves into sperm, and stop their tails from wiggling.
If a sperm can't swim, it has no chance of naturally fertilising a woman's egg.
The breakthrough made by British scientists could be the key to rendering men temporarily infertile.
It might become possible for them to pop a pill that stops their sperm from swimming.
The key advance raises the prospect of a fast-acting pill or a nasal spray that a man could take hours or perhaps just minutes before sex.
Researchers believe its effects would wear off within days, meaning he would again be fertile.
By contrast, women are typically advised to stop taking the Pill weeks or even months before trying to conceive.
Last night, family planning experts said a reversible male contraceptive could benefit millions of couples where the woman cannot take the Pill for medical reasons.
And it would save men from being trapped into having children they did not want, they added.
With global sales of contraceptives topping 13 billion a year, the market for a reversible male contraceptive could be huge, but some cast doubt on the idea of a male pill, doubting women will trust men to take it.
Lead researcher Professor John Howl, of Wolverhampton University, last night described how effective their sperm-stopping agent had been in lab tests.
He said: "The results are startling and almost instant."
When you take healthy sperm and add our compound, within a few minutes the sperm basically cannot move. Male infertility is often the result of poorly moving sperm, called low motility.
Using this, the Wolverhampton team, together with Portuguese researchers, made a compound called a cell-penetrating peptide, which gets inside sperm and brings them to a standstill.
Prof Howl said: "This is a totally unique approach nobody else has ever done this before. Peptides are short chains of amino acids that can influence how human cells work."
They occur naturally but biochemists can make synthetic versions.
Prof Howl and colleague Dr Sarah Jones first demonstrated that certain cell-penetrating peptides could smuggle themselves inside sperm cells, like a Trojan horse.
Then they joined forces with IVF experts at Aveiro University in Portugal, who had identified the crucial protein responsible for putting the wiggle into sperm tails.
The two universities created a bespoke peptide which turns that protein off.
They tested the approach in the petri dish on bovine and human sperm, with similarly impressive results, and will soon publish their results.
Prof Howl said they hoped to start live animal tests in two to three years, thanks in part to a 175,000 Portuguese grant.
It typically takes three to five years to bring a new drug to market after animal trials, so the final product could be available as early as 2021. Prof Howl said it was too early to say if the end result would be a pill, a nasal spray or a sub-skin implant, but they were all possibilities.
John Guillebaud, emeritus professor of family planning and reproductive health at University College, London, said a reversible male contraceptive would be of enormous benefit to many couples, such as those where the woman cannot take the Pill for medical reasons for instance due to migraine with aura or an increased risk of blood clots.