London mayor Boris Johnson is on a "collision course" with the government over airport expansion, a Heathrow anti-expansion campaigner has said.
The government scrapped plans for a third runway and sixth terminal at Heathrow when it took office in May.
But John Stewart, of Hacan ClearSkies, claims the mayor is to argue for a new hub airport in the South East.
The mayor's office denied Mr Johnson was at odds with ministers and said he was speaking up for London.
Mr Stewart said he was told about the plans, due to be released in a report on Tuesday, by the mayor's aviation adviser.
The report comes as Mr Johnson joins business leaders in London to discuss the need for increased aviation capacity in the capital and south-east England.
Mr Stewart said: "Boris Johnson is on a collision course with the government over basic airport policy."
Denying this, a spokesperson for the mayor said: "The government has announced that it is developing a new sustainable aviation policy and wants to hear people's views.
"That includes the mayor speaking up for London.
"No. 10 and the DfT are aware of the seminar."
In October the government also ruled out proposals to build a major airport in the Thames Estuary or Kent to increase flight capacity for London.
But Mr Stewart claims the report will make the case for a hub airport, most likely on the Kent coast or offshore from Kent and Essex.
He said the mayor will argue that without such expansion London will lose out to other cities and businesses will relocate to other cities.
He added: "I don't think there is an economic case for expansion at London airports.
"If you put all London's airports together - Heathrow, Gatwick, Stansted, London City and Luton - you have more airport capacity than any other city in the world."
As chairman of Hacan ClearSkies, Mr Stewart said the group neither supported nor opposed the plan at this stage.
He said: "Our feeling right now is it's better for us to look at capacity that's already in London before embarking on a hugely expensive project which in recessionary times may never see the light of day."