Heathrow has mapped out how it will tackle one of the UK’s most complex motorway upgrades while keeping traffic flowing on the country’s busiest stretch of road.

The airport’s third runway scheme will need a brand-new section of the M25 to be built, complete with a tunnel and new bridges, at Junction 14 near Heathrow.

Rather than rebuilding in place, Heathrow will construct the new motorway 130 metres to the west of the existing road to avoid traffic chaos.

This “offline” approach means most of the heavy works will take place away from daily traffic.

Once complete, traffic will be switched from the old alignment onto the new during a series of planned overnight closures.

The re-engineered stretch will be wider and safer, with new link roads to untangle weaving movements between the M25 and M4.

Heathrow said the whole scheme to ease traffic congestion will be 100% privately financed.

Alternative options – such as placing the new runway over the M4 spur – were rejected for creating greater disruption, more demolition, and exposing more homes to aircraft noise.

They would also clash with the Heathrow Express and Elizabeth Line tunnel beneath the spur, making construction highly disruptive for both road and rail users.

Heathrow’s surface access director Sophie Chapman said: “Heathrow expansion is a once-in-a-generation opportunity to upgrade our connectivity in the air and on the ground.

“One of the legacies of our plan will be a 100% privately financed upgrade to one of the busiest stretches of the M25. We have a tested plan for building next to the current motorway that will keep motorists on the move during construction.”

Building runways over motorways is not new, with similar schemes successfully delivered in Frankfurt and Paris Charles de Gaulle. Heathrow said its plan is the only viable solution that balances aviation capacity with minimised road disruption.

The proposals were submitted to Government this summer, and Heathrow will now work with National Highways to finalise the design and phasing of works.