Moments earlier commission president Jose Manuel Barroso had been seen leaving, accompanied by his spokesman Johannes Laitenberger and other officials. He returned at around 6.15pm to thank the firefighters after they had finished their job.

"I came here just to say thanks to the firefighters of Brussels, they were brave, responsible, very professional and quick. Now everything is under control and we are doing business as usual in the neighbouring building. We have to draw a lesson from this, there will be an inquiry," the commission chief said in front of rolling TV cameras, while shaking hands with the head of the firefighting team.

He also personally thanked a few dozen tired, sweaty and smoky firefighters as they peeled off their helmets and headed home. "It's nice to have personally met him. [Before] I only saw him on TV," one of the firefighters told this website.

He explained that the main fire broke out in the electric shaft on level zero, but the team first went to level 13, where Mr Barroso's office is located. "We put it out there, but the heat kept on coming from beneath, so we went one floor down and so on until we spotted it on the ground floor."

The cause of the fire is still unknown.

The firefighting team used water cannons and fought the flames for about two hours. "It's a good question why the sprinklers did not go off," the firefighter said.

Repair work to the building's electric and computer wires is likely to take several days, with staff warned they might not be able to get back to their desks until Tuesday afternoon.

The building houses all 27 of the EU commissioners as well as the press-handling service and several hundred officials. But Mr Laitenberger said it was impossible to say exactly how many were in the building at the time of the outbreak.

Internal market spokesman Oliver Drewes told EUobserver he was in his office on the third floor when he smelt burning and knew something was wrong.

Journalists working in the commission's pressroom - known as "the bunker" - in the basement of the building noticed a steady build up of smoke over a 20 minute period prior to the evacuation.

The building's smoke detectors failed to go off until very late into the events, and only after people had already been told to leave the building.

The Berlaymont has been dogged by controversy in the past, remaining closed at great expense from 1991 to 2004 due to the discovery of asbestos in the building.