M25 could become Britain's first 14-lane motorway under Heathrow expansion plans

The airport wants taxpayers to bankroll £1.2bn of road and rail improvements. Photo: PA Wire

The M25 is set to become Britain's first 14-lane motorway if a third runway is built at Heathrow Airport.

The airport wants taxpayers to bankroll £1.2 billion of road and rail improvements including a 600m motorway tunnel under the new runway.

The M25 would be widened to 14 lanes on a two-mile section from Junction 14 next to Terminal 5 to the junction with the M4.

The proposal is likely to provoke a furious reaction from environmental campaigners.

The motorway plan was revealed as Heathrow bosses made their final submission to the government's airports commission.

Read: Gatwick expansion plans 'cheaper and more beneficial'