The UK government has approved Heathrow’s plan to build a new third runway, which involves moving the M25 motorway, reviving a long-running, highly controversial expansion project aimed at boosting capacity at the UK’s busiest airport.

Two competing proposals had been under review — one submitted by Heathrow Airport and another by the Arora Group. Heathrow’s plan called for a new runway of up to 3.5 km (2.2 miles) in length, along with the construction of a new road tunnel beneath the airport. Arora’s alternative suggested a shorter, less expensive runway that would avoid any need to alter the M25 motorway. A definitive ruling on whether the third-runway project will proceed is still several years away. Once that stage is reached, Heathrow is expected to file its planning application, with the government aiming to issue a final decision by 2029. The whole project, which is expected to cost GBP49 billion (US$64.8 billion), includes:

• The new runway, which Heathrow says will increase capacity to 756,000 flights and 150 million passengers a year. It currently serves about 84 million.
• a new terminal called T5X, expanding Terminal 2, and three new satellite terminals. It would close Terminal 3;
• enhancement of local rail connections and improvements to Heathrow's bus and coach stations; and
• diversion of the M25, which would involve a new road tunnel under the airport, and widening the motorway between junctions 14-15.