A consortium behind ambitious plans for a floating airport in the Thames Estuary has questioned why it was excluded from a shortlist drawn up by the commission investigating where new runways should be built.

The consortium behind the £47bn London Britannia Airport has written to the Davies Commission to complain about not being shortlisted in December.

It has also queried why the Isle of Grain scheme – an onshore project – remains under consideration.

In a letter to the Davies Commission, the architects Gensler – part of the consortium named the Thames Estuary Research and Development Corporation (Testrad) – said its version of a floating hub airport had “far greater merit” than the Isle of Grain project.

It has told Davies that “the dismissal of our scheme as not credible was a considerable surprise to both to us and those bodies with which we were engaged”.

It added: “The London Britannia proposal has far greater merit as a location than the Isle of Grain in any guise.”

Signed by Testrad head, economist Bridget Rosewell, the letter lists ten questions Davies should answer including how the commission decided the Foster’s plan “might constitute a ‘credible option’ [while] other estuary options would not be credible”.

In an interim report, the government-appointed commission shortlisted three options to extend Heathrow and Gatwick and announced that it would investigate the proposal for a hub airport on the Isle of Grain devised by Foster and Partners.

Testrad’s proposed estuary airport involves six fixed runways built in the Thames estuary, linked by a tunnel to Ebbsfleet in Kent and then on to London via High Speed One.

The consortium had argued it could be built in seven years, thanks to lower planning restrictions in the estuary.

The Davies Commission says it will make a final decision over the Isle of Grain scheme, which has been backed by Mayor of London Boris Johnsn, in the autumn.