Image copyright Fosters and Partners Image caption An artist's impression of the airport proposal on the Isle of Grain in the Thames Estuary

A plan for an island airport in the Thames estuary will be rejected, the BBC understands.

In response, an adviser to London mayor Boris Johnson, Daniel Moylan, said if true, it is "sadly short-sighted".

Known as "Boris Island" because of Mr Johnson's backing, it was one option being considered by the Airports Commission on how to expand airport capacity in the UK.

An official announcement is expected on Tuesday.

"Airports policy has been stalled for nearly five decades, ricocheting like a billiard ball between Heathrow and Gatwick," said Mr Moylan, aviation adviser to Mr Johnson.

"We have only one opportunity to break out of that but it seems the Commission has taken us back to the same old failed choice."

He added that the final decision would rest with the government. "The key question now is whether the Airports Commission will play much of a role," he continued.

Earlier, the Financial Times reported that Whitehall sources had confirmed the "Boris Island" plan would be ruled out.

Image copyright Foster and Partners Image caption The airport plans on the Isle of Grain were designed by Lord Foster's architectural firm

It would have led to the development of a four-runway airport on the Isle of Grain in the Thames Estuary.

The commission, led by Sir Howard Davies, was set up by the government to consider ways of expanding the UK's airport capacity. Its final report is expected next summer, after the general election.

The decision to eliminate Boris Island as an option would leave the commission with three alternatives: Adding a third runway at Heathrow, lengthening an existing runway at Heathrow, and a new runway at Gatwick.

Analysis: Richard Westcott, BBC transport correspondent

Sir Howard Davies' decision is unlikely to surprise many people.

A few months ago he told me that the Thames Estuary scheme had much higher hurdles to jump over than its competitors at Heathrow and Gatwick.

He said the construction challenge was "massive", and that he was concerned about the impact on the environment, the cost of getting 150m passengers to and from the site (you would have to build new roads and rail lines), and the effect it would have on Heathrow, which may have to close.

For several months now he has been analysing the detail and in the end he has decided it's not worth all that cash.

So now the focus is very much on Heathrow, which boasts two of the three schemes on the Airports Commission short-list.

Expanding Heathrow is a politically toxic idea. The whole point of setting up a Commission was to delay any decision until after the general election.

It's just possible that the politicians will be pressured into declaring a favourite before then.

'Maximise links'

"By excluding the Thames Estuary airport option, the Airports Commission has made the right choice on the basis of robust evidence", said Chris Richards from the manufacturers' organisation the EEF.

Before the decision about "Boris Island" emerged, the CBI business lobby group said that a single, larger-hub airport where passengers can transfer to a range of destinations was "critical" to the UK's long-term economic growth.

The CBI urged the commission to ensure its decision "maximised links across the UK" and made the "best use of existing capacity".

"While no-one can predict the future of air travel, the track record shows that it tends to be hub airports that deliver the new connections to emerging markets that we desperately need," said CBI deputy general Katja Hall.

Heathrow said this meant that the CBI backed its expansion but Gatwick said hub capacity could be achieved by switching some traffic to its airport, freeing up space at Heathrow.