Firefighters were called to reports of a methane alarm sounding in a tunnel on Tunnel Road East at Heathrow Airport.
A gas pipe running through the tunnel was isolated and crews carried out a sweep to check for any elevated readings.
Firefighters worked with airport staff and a hazard zone was been put in place as a precaution. The tunnel’s extraction system was activated and the tunnel was closed.
Station Commander Jim Berry said: "Crews worked hard with partners to resolve the incident, however both tunnels were closed and there was significant congestion in the area.
"We asked drivers to take this into account when planning their journeys."
The Brigade was called at 1319 and the incident was over for firefighters by 1709. Two fire engines from Heathrow and Hayes fire stations and a fire rescue unit from Wembley Fire Station were in attendance.