[Benjamin]

What we know so far
published at 19:34


At 17:01 BST this evening, the London Fire Brigade (LFB) was called to Heathrow Airport. LFB said it was responding to a "possible hazardous materials incident".

Terminal 4 was evacuated, a spokesperson from the airport said, and passengers were seen waiting outside.

National Rail said trains had been unable to call at the Terminal 4 stop as emergency services were "dealing with an incident".

What we don't know is what the specific issue is, nor what caused it. It's also unclear what the knock-on effect on air traffic will be - some flights appear to be going ahead, while others in various terminals are showing delays.