Boris Johnson's controversial proposal to replace Heathrow with a new 24-hour airport in the Thames estuary could bring with it a number of environmental benefits, according to the Mayor's Office.

Earlier this week, Johnson confirmed he was to commission a viability study on the proposals which would see the planned expansion of Heathrow scrapped in favour of a new four runway airport built on an artificial island in the Thames estuary and modelled loosely on Hong Kong airport.

The new airport, which could ultimately be expanded to six runways and replace both Heathrow and Gatwick, would be connected to London via a 35-minute high speed rail link and would also link with the Channel Tunnel, allowing passengers to travel direct to Brussels or Paris within two hours.

Johnson, who has described Heathrow as "a planning error of the 1960s", said that it was not impossible to move the UK's largest airport, arguing that a new airport would have many advantages over an expanded Heathrow.

A spokesman for the Mayor said that no work had yet been done to assess the potential environmental impact of a new airport, adding that the official feasibility study had not yet commenced.

However, he said that Johnson was opposed to Heathrow expansion on a number of grounds, some of which related to the environment.

"We want to look at whether environmental pressure [arising from Heathrow's expansion] should be put on the densely populated West of London when there could be a solution to the east," he said, adding that it "was safe to say" that moving the airport into the estuary would help cut noise and air pollution levels.

Plans for a third runway at Heathrow have prompted concerns from both the EU and the Environment Agency over fears that the expansion would result in EU air pollution rules being broken, while opposition to increased noise pollution levels have been at the root of many local residents' opposition to the proposals.

It is also hoped that the new airports links to the Channel Tunnel could help cut numbers of short haul connecting flights to the continent by providing passengers with fast links to Northern Europe.

However, Ben Stewart of Greenpeace slammed the proposals, arguing that any environmental benefits associated with a new airport would be negated by an increase in the number of flights.

"New runways, are new runways, wherever they are located," he said. "These proposals would not deliver net environmental benefits as we are still talking about a massive expansion in flights at a time when we need to be bringing emissions down."

He added that instead of investing in expanded airport capacity, the UK should be looking to increase funding for the rail network and other forms of low carbon transport.

"The biggest story in the aviation sector at the moment is contraction, while the government is planning new runways based on the assumption that oil prices will stand at $53 per barrel," he said. "We shouldn’t be building runways to nowhere and should instead be encouraging people on those 100,000 flights a year out of Heathrow that could be served by rail, to get on the train."