Scientists have made a breakthrough that could be the key to developing a male contraceptive pill.
The discovery uses a peptide that changes the way human cells work, "switching off" sperms' ability to swim, to render men temporarily infertile.
Scientists hailed the results as "startling - and almost instant". It raises the prospect of a fast-acting pill or a nasal spray that a man could take hours or even minutes before sex.
Women are typically advised to stop taking the pill weeks or even months before trying to conceive.
But researchers believe the effects of a male pill would be almost instant and wear off within a matter of days.
Lead researcher Professor John Howl, of Wolverhampton University in England, said the new compound, made in the lab, had shown immediate results.
"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," he said.
Working with scientists from Portugal, the team made a compound called a cell-penetrating peptide.
Peptides are short chains of amino acids which influence how human cells work. They occur naturally but can also be created synthetically.
The breakthrough came after scientists in Wolverhampton demonstrated that particular peptides could penetrate sperm cells. Then fertility experts at Aveiro University in Portugal, who had identified the protein that drives sperm to swim, created a bespoke compound that turned the protein off.
The approach was tested in the lab on bovine and human sperm, with live animal tests due to start within three years.