Britain’s foremost aviation official has warned that building a new airport in the Thames Estuary would cause problems in the skies over London.

Richard Deakin, boss of the National Air Traffic Service, said although four new runways in the southeast may be able to function on their own a new airport would not be able to co-exist with London City and Heathrow.

Mr Deakin said that although a new airport would not necessarily cause the closure of Heathrow it would force a dramatic cut in capacity. He explained that this would be a difficult conversation to have with airlines including British Airways which is based at Heathrow. He added that there was also substantial investment going on at the UK’s busiest hub.

The construction of Boris Island has been estimated at £70 billion. Mr Deakin said that he had not yet been contacted by the Mayor of London over the feasibility of the proposal. The government has already said that a third runway at Heathrow is not one of the options it is looking at in terms of creating more capacity.

Ministers are due to publish an Aviation Policy Framework by the end of the summer. However, experts suspect that it will not touch on sensitive issues including aviation capacity but on broader subjects such as cutting airlines’ carbon emissions.