Rail services in southern England were hit by a fresh strike on Saturday and fog caused the cancellation of dozens of flights at London's main airport.

The RMT union launched a three-day strike on Southern Rail, which runs commuter services from the south coast into London, as part of a long-running dispute over plans to downgrade the role of the train guard.

Southern Rail said services were severely disrupted by what it called "pointless" action, with many routes cancelled and others running a skeleton service.

The dispute is focused on plans to introduce driver-only operated trains.

Currently a guard is required to open and close the doors, but the train operator says the driver should be able to do this. Union leaders says such a move would put passenger safety at risk.

There was also disruption in the air on Saturday as 40 flights were cancelled at London's Heathrow Airport, the knock-on effect of heavy fog the previous day when 140 flights were cancelled.

"Aircrew and aircraft are in the wrong place," a spokesman told AFP, adding that although the fog continued into Saturday, it was expected to lift mid-morning.

There were also a "handful" of flights cancelled at London Gatwick, as well as a dozens of flights delayed due to fog elsewhere in Europe, a spokeswoman for the airport said. afp