Relocating protected habitat affected by an international hub on the Isle of Grain will cost up to £500m, says the firm behind the airport idea.

In a detailed response to a call for evidence by the Airports Commission, architect Foster and Partners estimate 18sq km out of a total of 35sq km of land covered by a future airport is conservation area.

It predicts 37% of special protection area in the Thames Estuary and marshes would be lost overall if an airport on Grain is built.

The report says it will compensate for this lost land with 2,000 to 7,000 hectares of habitat elsewhere, at a cost of between £100m and £500m.

This is likely to be found along the north Kent and Essex coastlines, with an RSPB study highlighting 34 potential sites in the South East and another 100 in East Anglia.

The feasability study also says the airport would be designed to minimise the risk of bird strikes and says the area is not a major flight path for birds.

It adds the majority of birds which live in the area will not be disrupted by higher noise levels.

It says: “It is accepted that there would be direct and indirect effects.

“An appropriate degree of habitat creation and/or restoration is feasible to enable the likely significant effects to be compensated to a level where the ‘overall coherence of the network’ is maintained.

“There are sufficient intertidal habitat creation opportunities in the north Kent area or further afield within Essex and the south and east of England.

“Ideally, compensation locations will have comparable conditions to those at the Thames Hub site (eg salinity, hydrodynamics) to maximise the likelihood of successful compensation provision.”

The feasability study says the hub airport will cost £15bn compared to £11bn for a third runway at Heathrow.

Making its economic case, it says the airport will be able to deal with 1.2m annual transport movements, compared to 740,000 at a three-runway Heathrow. It would be able to handle 150m passengers a year.

This would serve 191 destinations, compared to 126 at Heathrow, operating 24 hours as opposed to 17.

With a 26-minute high-speed connection to London St Pancras, the airport could help businesses contribute a cumulative £73bn to the UK economy to 2050.

"Britain needs an effective long-term solution, not the usual short-term fix that is Heathrow’s proposed third runway..." - Lord Foster

The construction of the site and surrounding infrastructure is expected to provide an average of 26,400 local direct, indirect and induced jobs each year over a 10-year period.

Once opened, it is expected the site will create 110,000 direct jobs and support a total of about 134,000 jobs, expected to be worth £107bn by 2050.

Lord Foster said: “Since the Airports Commission submission a year ago, the need for increased airport capacity has become even more urgent.

“Britain will need to plug-in to the network of major new hub airports being built and planned around the world, for example in Istanbul, Doha, Dubai, Beijing and Mexico City. If you cannot fly through London and connect to the destinations these new hubs offer, then trade will simply go elsewhere.

“It is time to get serious about the issue of airport capacity. Britain needs an effective long-term solution, not the usual short-term fix that is Heathrow’s proposed third runway.

“Rather than boosting growth, expansion at Heathrow would have the opposite effect. The unacceptable levels of risk, noise and pollution would threaten London’s leading reputation as a world city.

“Only long-term thinking will properly serve the demands of our future generations.”

The RSPB believes the damage to the environment could be the undoing of the £15bn Grain airport.

“By the time they can build an airport, we will be retiring. It is not the solution...” - RSPB's Rolf Williams

Communications manager Rolf Williams – who lives in Rochester – said: “If this all goes through and the airport design at Grain is consented, then they cannot start building it until they have created a 100sq km of tidal estuary and marshland.

“That has never been done before anywhere in the world on that scale and if they try to do it then there is no question it will take many decades to achieve before it satisfies that legal requirement.

“By the time they can build an airport, we will be retiring. It is not the solution.”

However, campaigners in favour of the airport maintain it is key for prosperity in the area.

Demand Regeneration In North Kent (DRINK) founder and director Clive Lawrence said: “This submission gives north Kent even more reason to welcome the project as it confirms the huge potential the airport has to change the lives of local people for the better.

“More well paid jobs and investment in local public services will mean our area will be the envy of the rest of the country. Even the birds will benefit with new, purpose-designed habitats.

“The details also put paid to the propaganda from the antis about the cost of the enterprise.

“This represents the final call for local politicians to come to their senses and support the best opportunity our community has ever had to give our grandchildren a golden future.”

To read the full feasability study, go to www.fosterandpartners.com