The central London offices of the Civil Aviation Authority had to be evacuated yesterday due to a fire in a next door service tunnel.
The building in Kingsway, where around 200-300 CAA staff are based, remains closed today (Thursday).
Firefighters are still battling the blaze, which has been declared a major incident by emergency services.
It is believed the fire, which caused a major power cut, was caused by faulty cables.
In total, around 2,000 people were forced to evacuate office buildings in the area, which also include MSC Cruises’ Holborn offices and call centre.
The cruise line tweeted today: "Electrical fire in holborn still impacting MSC Cruises UK as we still have no power. Please use online alternatives. Our apologies."
The CAA said its switchboard is open as usual and its Gatwick office, Aviation House, is also open.
London staff are operating from the Gatwick office or from home.
A spokesman said he didn’t know if the office building had been damaged or what impact the closure would have on CAA operations. Tuesday is the renewal deadline for ATOL licences.
"We will keep our stakeholders informed of the situation and will take our lead from the emergency services," he said.
The CAA’s website said: "If you need to contact any of our London-based staff, please call the switchboard in the first instance," it said.
"CAA staff based in London will be kept updated through their line manager, email and the CAA intranet."
