Heathrow unveils “offline” M25 tunnel plan to support third runway expansion

Heathrow Airport has detailed its strategy for one of the most complex motorway upgrades ever attempted in the UK, constructing a new section of the M25 to accommodate its proposed third runway. The reconfiguration involves building a tunnel and new bridges at Junction 14, near Heathrow, approximately 130 metres west of the current alignment.

By adopting an “offline” construction approach, engineers will perform the majority of heavy works away from live traffic. This method significantly reduces disruption along one of the busiest motorway corridors in the country. Upon completion, traffic will be redirected from the existing M25 to the newly constructed alignment through a sequence of carefully planned overnight operations.

The upgraded section will feature wider lanes, improved safety features, and new link roads that separate through traffic from vehicles connecting to the M4. This configuration aims to reduce congestion and eliminate weaving movements which is a common cause of motorway accidents.

The project is entirely privately financed, forming part of Heathrow’s long-term infrastructure vision to enhance both air and ground transport connectivity. Alternative proposals, including placing the runway over the M4 spur, were ruled out due to potential large-scale demolition, increased noise exposure, and interference with existing rail tunnels serving the Heathrow Express and Elizabeth Line.

Heathrow’s Surface Access Director described the project as a “once-in-a-generation opportunity” to deliver an efficient transport solution while maintaining road access during construction. The airport continues to collaborate with National Highways to finalize design and construction phasing, ensuring compliance with national safety and operational standards.

Check out the following video for more information on the major M25 expansion.