Passengers travelling to Heathrow Airport are facing chaos on the road today as a tunnel leading to Terminals 2 and 3 has been closed due to a gas leak.
The M4 has been closed in both directions between Junction 4 and Junction 4A while an inspection is carried out.
The airport has advised passengers to avoid travelling by road and use the rail network and underground instead.
Heathrow said travellers can also use the free terminal transfer service from Terminals 4 and 5.
While the incident is ongoing, cars have been unable to leave the airport's car park which is completely gridlocked.
Several buses - including 105, 111, 285, 278, A10, X26, X140, U3 - are on diversion.
Just after 2.30pm, National Highways tweeted: "The #M4 in #GreaterLondon is closed both ways J4 - J4A for @HeathrowAirport due to an off-network incident. @metpoliceuk along with Heathrow Airport crews are in attendance.
"Traffic heading to Heathrow Airport should continue to M25 J13 and follow the signs for Terminal 4."
A spokesperson for Heathrow Airport said: "The tunnel to Terminals 2 and 3 is partly closed while our teams and the Fire Service carry out checks.
"There is congestion across Heathrow’s road network and passengers travelling to Terminals 2 and 3 are advised to avoid travelling by road and to instead use the rail network and underground.
"They may also use the free terminal transfer service from Terminals 4 and 5.
"If you are travelling by road, please allow extra time for your journey. We apologise for any inconvenience caused."
A Metropolitan Police spokesperson said: "Officers were called at 13:15hrs on Tuesday, 4 October to the M4 near Heathrow airport.
"The East Bore and West Bore tunnels had triggered gas alarms.
"The roads have been closed as a precaution. LFB and Gas Board are attending."
Just after 4.30pm, Heathrow Airport added: "There is congestion across Heathrow’s road network due to an issue in the tunnel leading to Terminals 2 & 3.
"Part of the tunnel is closed while we conduct safety checks."
