The decision to keep open the option of a four-runway hub airport in north Kent has been defended by the man leading the government review into aviation capacity.
Sir Howard Davies told MPs he needed more time to assess whether or not the scheme was viable, but a decision would be made by the autumn.
He also shrugged off a verbal broadside from Mayor of London Boris Johnson, who at the weekend said the commission's treatment of the estuary option had been "gloopy, tangled and quite labyrinthine".
Economist Sir Howard Davies is conducting an aviation review
Appearing before the transport select committee yesterday, Sir Howard said he had been surprised by the mayor's comments, but would "rise above this vulgar abuse".
He told the committee: "I completely dismiss what the mayor has said about our report. It's completely at variance with anyone else.
"There are a whole variety of arguments that are difficult to weigh up against the other schemes. The environmental arguments are quite difficult to assess..." - Sir Howard Davies
"As far as the mayor's concerned, we don't seem to have produced the answer he wants us to produce. We will proceed with our analysis in the way we've been asked to do.
"I don't think it's at all helpful that he uses this rather colourful language, but I guess it's a matter of style.
"I don't think it illuminates the debate to do so. But we will not react, we will simply press on."
On the decision to include the Isle of Grain option in an interim report, he said his team had decided it needed more time to examine the case.
"There are a whole variety of arguments that are difficult to weigh up against the other schemes. The environmental arguments are quite difficult to assess. We eventually decided we did not have enough information to make a firm decision."
An aerial image of Lord Foster's proposed Thames Hub airport at Grain
He added if the Isle of Grain scheme was chosen as the best option, the government would have to take over Heathrow while it was wound down and closed.
"It would be the biggest planning decision that London would make in a generation or more," he added.
Boris Johnson wants an airport built in the Thames Estuary
The government has asked Sir Howard to deliver his final recommendations after next year's general election, but the commission could have produced a report this side of the 2015 election.
"There will be a significant capacity problem in the mid 2020s so a decision shortly after the election will be important," he added.
Heathrow has said it could install a third runway by 2026, with an estimated price tag of up to £18bn. Gatwick said it could have a second runway built by a similar date.