A fire engulfed a warehouse used by Emirates Airline at Heathrow Airport outside London on Friday, a fire brigade spokesman said, but flights from the busy British international air hub were not affected.

'A significant part of the building is alight,' the brigade spokesman said. He said more than 100 firefighters were tackling the blaze but it was too soon to say what had caused it.

Flights in and out of the airport, one of the world's busiest, were unaffected and there were no reports of any injuries, an airport spokeswoman said.

A gas cylinder explosion was believed to have started the fire in a warehouse used by Emirates and a number of other carriers, Boutrous Boutrous, head of media for the Dubai-based airline, told Reuters.

A statement from an Emirates spokesman in Dubai said its employees were safely evacuated from the Servisair Cargo warehouse.

Heathrow, west of London, is owned by Spanish builder Ferrovial.-Reuters