Tesco has launched an investigation after a viral video appeared to show two staff members engaging in a sex act in-store.

Voyeuristic teenage boys filmed the frisky duo getting hot and heavy in a back office of the Stalham branch in Norfolk, East Anglia.

The supermarket chain has not denied that the clip was filmed at the Stalham store, saying: "We are taking this issue very seriously and have launched an immediate investigation."

In the video, a woman wearing what appears to be a Tesco uniform straddles a desk while a man sitting on an adjacent chair performs a sex act on her.

The pair appear to be in a back office, but are clearly visible through a large glass window, which the teenage boys exploited with a camera phone.

"She likes that. She's going for a pay rise. F***ing hell," says one of them, as they spy on the couple.

The video quickly went viral on WhatsApp and other social media platforms, including Twitter, according to The Sun.