The plan was first mooted during his election campaign, but will now have work done on it by Mayoral advisers as part of a review of London’s airports, and their future expansion.
A spokesperson for the mayor said Mr Johnson "wants to undertake a study to establish the feasibility of the idea once and for all and is in the process of examining the best way to take the study forward."
Anti-Heathrow campaigners HACAN (Heathrow Association for the Control of Aircraft Noise) seem to think it’s not a particularly feasible alternative – they told the BBC "the market in the South East couldn’t support two major hub airports, it would need to be either Heathrow or an off-shore airport. But who is going to invest in a new airport with Heathrow just the other side of London?"
And environmental capmapigners weren’t happy either – the RSPB points out that the Thames Estuary is used by many migrating birds, and that building a 24 hour airport on the Isle of Sheppey could destroy their habitats.
By the end of this year the government is due to make a decision on the proposed expansion of Heathrow.
Operator BAA has been stressing the need for expansion saying that Heathrow is "jam-packed" and needs a third runway to remain competitive globally.
There is no mention in the Mayor’s statements of the interesting proposal to reduce the need for runway expansion by linking Heathrow more effectively to the rail network proposed by Arup, and reported on lookwest earlier this year. Perhaps there should be.