Armed robbers have pulled off an audacious heist at Brussels airport, making off with diamonds worth around $50m.

The robbery was “one of the biggest” ever, a spokeswoman for the Antwerp World Diamond Centre said, adding that the diamonds were “rough stones” being transported from Antwerp to Zurich.

Earlier reports had put the value of the shipment at about £300m.

The gang of eight  thieves hit a Brinks transport truck on the airport tarmac as its contents were being loaded onto a Swiss aircraft bound for Zurich, according to Belgian state TV.

An airport spokesman said the robbers cut a whole in a perimeter fence but was unable to explain why it had been so easy for them to avoid security.

“We abide by the most stringent rules,” Jan Van Der Crujsse said.

Brussels prosecutor’s spokeswoman Anja Bijnens said the masked men drove two vehicles up to the plane.

Reports said no shots were fired and no-one was injured as the heist was completed in a matter of minutes.

Police found a burnt-out van close to the airport on Monday night but are looking for further clues.