South Africa's Rugby World Cup heroes are stripping down to their underwear on their social media accounts to raise awareness about testicular cancer.

It is a challenge thrown to them by team-mate Faf de Klerk, famous for celebrating victory against England while wearing just his pants.

He took to Twitter with his #FafChallenge campaign drive.

The campaign urges men to get "ballsy enough to check on their crown jewels" for signs of testicular cancer.

Though not classed as one of the "big five cancers affecting men in South Africa" by the Cancer Association of South Africa, it is particularly common in men aged between 15 and 49.

Self-check challenge

The challenge is a partnership with Cipla South Africa, a pharmaceutical company which runs a website that offers guidelines on how to self-examine unusual lumps in testicles, and offers a factsheet on testicular cancer.

A spokesperson for the company is quoted by the News24 website as saying that "they were hoping for the same talkability about these important self-checks that women had established about regular breast checks".

The campaign has seen Springboks captain Siya Kolisi posing in his pants on his Instagram page. Other team-mates doing the same include Makazole Mapimpi, Cheslin Kolbe, Damian de Allende, Malcolm Marx and Jesse Kriel.

How to check your testicles

A good time to check your testicles is after a warm bath or shower, when the skin is relaxed.

Cup your hand under them and check for swelling and lumps.

Roll each testicle between your finger and thumb and feel the weight.

The UK's National Health Service (NHS) says most men's testicles are around the same size, although it's common for one to be slightly bigger than the other or hang lower.

There might be something wrong if you find a hard lump on the front or side of a testicle, if a testicle is swollen, or if there's pain or discomfort in a testicle or in the scrotum (the sack that holds the testicles).