An airport in the Thames Estuary would involve spending billions on widening the M25 and other routes and a huge compensation bill to close Heathrow, it's been revealed.

Four reports by the Airports Commission on the feasibility of an Inner Thames Estuary airport looked at the environmental impact, the operational feasibility, the socio-economic impacts and surface access.

The report on the environmental impact found the cost of creating replacement habitats for the hundreds of thousands birds could exceed £2bn.

The second report published today covered the operational feasibility, from flood risk and bird strikes to the removal of SS Richard Montgomery ship wreck and the LNG facility on the Isle of Grain.

The wreck of the SS Richard Montgomery is 5km from the airport perimeter. The 1944 munitions vessel contains explosives that have been deemed too high risk and too costly to remove, potentially requiring evacuation of the local area for weeks or even months.

The report concluded that although the topics addressed in the report offer “significant but perhaps not insurmountable challenges and risks” on their own, when considered altogether they present “a substantial risk that would incur large costs, in the order of billions of pounds, to appropriately manage”.

It also looked at the attitudes towards the airport and found that “with the exception of Kent businesses, all stakeholders considered the Estuary airport scheme to carry significantly more risk than opportunity”.

The report on the socio-economic impact of an estuary airport concluded that “Heathrow will need to close in order to make an inner estuary Thames Estuary airport commercially viable”.

The owners of Heathrow would expect to receive between £13.5bn and £21.5bn in compensation.

Locally, however, an airport would generate 98,000 additional jobs by 2030 across six local authorities, a 23.5% increase in the current forecast. But the report warned that this may be constrained by housing availability, labour supply, availability of land and access.

The final report on surface access considered the possible road and rail routes needed for an estuary airport.

There are four viable rail options varying in cost from £9.8bn to £26.9bn, but in terms of engineering feasibility “there would be significant challenges to overcome to provide a successful rail package”.

The M25, M2 and the A2 would all have to be widened at a cost of £10.1bn to £17.2bn.

Improving the surface access to the peninsula would have “wider ranging impacts extending long distances from the proposed airport development”.

There are many internationally designated sites in the area which would be affected including Sites of Special Scientific Interest (SSSI), Areas of Outstanding Natural Beauty (AONB), Air Quality Management Areas (AQMA) and historic sites.

The airport issue is in a crucial phase with the Davies Commission set to rule on London Mayor Boris Johnson’s now-favoured Foster airport blueprint on Grain in the autumn.