A DECISION on whether to add a Thames estuary "Boris island" option to an official airport expansion shortlist will be made this autumn.

Chaired by former Financial Services Authority chief Sir Howard Davies, the Whitehall-appointed Airports Commission has already shortlisted Heathrow and Gatwick Airports as possible sites for a new runway by 2030.

The estuary airport plan, favoured by London Mayor Boris Johnson, failed to make the shortlist in last month's interim report but the commission promised to look further at the scheme.

Announcing the future work of the commission in an "appraisal framework" timetable, Sir Howard said a decision on whether to add the estuary airport option to the shortlist would be taken in "early autumn".

Addressing an aviation conference in London, Sir Howard said the estuary scheme, costing as much as £112 billion, presented "many challenges and risks", adding the cost looked "rather daunting".

An extra third runway at Heathrow was given the go-ahead by the Labour government in 2009, but scrapped by the coalition Government when it took power in May 2010.