Heathrow has defended plans for its new runway to be built over the M25 in the face of claims that accidents could be caused by drivers being distracted by aircraft landing.

Highways England has reportedly told the airport to introduce measures to “reduce driver distraction” on the affected section of the M25, the UK’s busiest stretch of motorway.

This could include lengthening the tunnel under the runway or simplifying the road layout to cut down on the number of decisions that drivers have to make.

Heathrow was also told to consider the “landing zone of aircraft”, suggesting they should avoid arrivals directly over the road itself.

The comments are made in a series of reports produced by the government-funded company into Heathrow’s third runway plan, which were obtained by The Times under freedom of information laws.

Highways England raised particular concerns about the risk of drivers being distracted by the sight and noise of aircraft landing in their direct vision as they enter the tunnel.

Runways and taxiways have been built over roads elsewhere in the world, such as Atlanta and Fort Lauderdale in the US and Paris Charles de Gaulle and Amsterdam Schiphol in Europe.

The airport wants to build a new two-mile runway directly over the M25, which widens to six lanes in each direction past the airport, carrying 220,000 vehicles a day. It is proposing to shift the motorway 150 metres to the west and lower it by seven metres in a tunnel while also raising the runway slightly. Construction could begin as early as 2021.

The full details will be published for public consultation in June.

Highways England said the project was “deliverable” but that there were “concerns about several aspects of the design”.

In a document delivered to Heathrow last March, Highways England said: “Consideration must be given to measures to reduce driver distraction as a result of the runway crossing the M25 and associated aircraft movements both on taxiways and take-off and landings. Measures which reduce decision making for motorists in this location should be considered.”

A Heathrow spokeswoman said: “The options we’ve proposed for crossing the motorway are tried-and-tested at other airports around the world. Highways England have reviewed our early design and confirmed that our plans are robust and deliverable.”