A man faces a hefty fine and a driving ban after being caught having sex with his girlfriend while speeding on a motorway in Norway, police have said.

Officers initially trailed the couple's car after noticing it was swerving from side to side and travelling at 33km/h (20mph) over the road's speed limit.

But they soon realised the erratic driving was due to the woman "sitting on the man's lap", a spokesman said.

After filming the exploit for evidence, they pulled them over at a rest area.

The 28-year-old man's punishment will be decided within the next week, but police said he was likely to face a fine of several thousand Norwegian kroner and a lengthy driving ban.

"Why they did it on a highway with such a high risk we don't know," Tor Stein Hagen, a superintendent with Soendre Buskerud Police District, told the AFP news agency.

"[The vehicle] was veering from one side to the other because the woman was sitting on the man's lap while he was driving and doing the act, shall we say," he added.

"He couldn't see much because her back was in the way."

Despite her prominent role in the incident, the 21-year-old woman was permitted by police to drive her lover home.