Heathrow Terminal 3 evacuated amid reports of fire alarms

It later said that the there was "no fire in T3 despite the alarm", and that passengers "will be directed into the terminal shortly". A series of social media users had posted messages and photos of hundreds of travellers exiting the building at the airport in west London.

The airport said earlier that the terminal had been evacuated and that it was investigating the incident as a matter of urgency. One Twitter user, Ben Smith, wrote: "London Heathrow Terminal 3 currently being evacuated. Fire Alarms going off. Lots of confusion."

Flights from the terminal appear to have been suspended. Other terminals are unaffected by the problems.

The source of the alarm still isn't clear, and passengers have complained that they had been left without any information from staff.

Terminal 3 is second only to Terminal 5 in terms of activity. It handles 50,000 passengers a day flying on a range of airlines, including Virgin Atlantic, Emirates, Singapore Airlines and some BA flights.

Around 250 flights arrive and depart at the terminal each day. The alarm was triggered at around 12.32pm, while the terminal was busy with passengers checking in for transatlantic flights, as well as services to the Gulf and Asia.

Those passengers were taken outside and were left waiting while the problem was addressed. Others were outside on the tarmac after being stopped from boarding planes.