Prostate cancer may be a sexually transmitted disease caused by a common yet often silent infection passed on during intercourse, scientists say – but experts say proof is still lacking.

Although several cancers are caused by infections, Cancer Research UK says it is too early to add prostate cancer to this list.

The University of California scientists tested human prostate cells in the lab.

They found a sex infection called trichomoniasis aided cancer growth.

More research is now needed to confirm the link, they say in the journalProceedings of the National Academy of Sciences (PNAS).

Trichomoniasis is believed to infect some 275 million people worldwide and is the most common non-viral sexually transmitted infection.

Often, a person will have no symptoms and be unaware that they have it.

Men may feel itching or irritation inside the penis, burning after urination or ejaculation, or a white discharge from the penis.

Women may notice itching or soreness of the genitals, discomfort with urination, or a discharge with an unpleasant fishy smell.

This latest research is not the first to suggest a link between trichomoniasis and prostate cancer. A study in 2009 found a quarter of men with prostate cancer showed signs of trichomoniasis, and these men were more likely to have advanced tumours.

The PNAS study suggests how the sexually transmitted infection might make men more vulnerable to prostate cancer, although it is not definitive proof of such a link.

Prof Patricia Johnson and colleagues found the parasite that causes trichomoniasis – Trichomonas vaginalis – secretes a protein that causes inflammation and increased growth and invasion of benign and cancerous prostate cells.