Many “lubricant” products that claim to be safe for male fertility could actually harm sperm, doctors have warned.

A study has found that three lubes marketed as ‘sperm friendly’ and sold in high street pharmacies for £20 a bottle can reduce motility – or their ability to swim in the right direction.

Despite the promotional health claims, the researchers concluded that these products may “in fact interfere with normal conception.”

The review also looked at two bizarre “natural” lubricants – olive oil and egg whites – that have become popular in certain corners of internet forums.

Pre-Seed, which retails for £18.49 per 40g tube at Boots, has been shown to reduce sperm motility.

Optilube, priced at £19.99 for 400g on Amazon, and Yes Baby, which costs £17.99 per 150ml bottle at Holland and Barrett, prevented sperm from being able to move completely.

Olive oil also stifled sperm’s ability to spread, while egg whites seemed to have no impact on their performance.

However, raw eggs can contain harmful bacteria – including salmonella – and experts don’t recommend using them during sex.

Traditional lubricants have been shown to damage sperm motility because they are too acidic and may contain harmful chemicals, including parabens.

This has led to an increase in demand for so-called “fertility-friendly” lubricants, although these cost up to four times more than the traditional product.

Users of forums like Mumsnet have also advocated the use of egg whites and other homemade products, despite the risks.

For the latest study, published in the International Journal of Gynecology & Obstetrics, researchers at the University of Pretoria in South Africa tested the three branded lubricants and the two natural lubricants on 60 semen samples.

The men were aged 18 and over and the samples were taken from a fertility clinic in Midstream, Gauteng Province.

The researchers used pipettes to divide the samples into six before adding 0.5ml of each of the five lubricants, while leaving one as a negative control sample.

Microscopic cameras were used to measure the speed and movement of sperm on glass slides, using a score recognized by the World Health Organization (WHO).

After two hours, semen mixed with olive oil and pre-seed samples had scores of one – directionless movement – ​​and Optilube and Yes Baby had scores of zero – no movement at all.

The sperm mixed with the egg white had a score of two, meaning it had slow or slow progressive motility.

Meanwhile, sperm not treated with lubricant had scores of more than two over the period – not statistically higher than egg white.

After 24 hours, the three branded lubricants and olive oil all had scores of zero, which means the semen was not moving at all.

The egg white and control groups had a score of one at 24 hours. All six samples were reduced to zero after 72 hours.

Lead author Johan Markram said: “This study is important because it highlights that these so-called sperm-friendly lubricants may actually interfere with normal conception.”

Women trying to conceive have started using egg whites more as a natural lubricant in recent years, with message boards calling them useful for getting pregnant.

Experts say it became popular because the type of vaginal mucus produced during ovulation by the body has the same name – egg white mucus – which may confuse people doing their own research.