The cost to London council taxpayers of promoting the controversial "Boris Island" airport scheme is to almost double to £5m.
The mayor is set to increase the current £3m budget to allow his officials to continue making the case for an estuary airport.
The scheme for a new international hub airport on the Isle of Grain in Kent failed to make the shortlist announced by the government's Airports Commission in December.
Additional runways at either Heathrow or Gatwick emerged as the favourites but commission chairman Sir Howard Davies agreed to take a second look at Boris Island.
Boris Johnson said the idea was still alive after it was "plucked from the waste paper bin."
He told ITV News: "Failure is not something we are currently contemplating"
London Assembly Labour member Dr Onkar Sahota accused the mayor of wasting Londoners' money.
"This is a pie in the sky idea which no one wants", he said