The case for Heathrow expansion is now "overwhelming", according to the man who led the government-commissioned review of airport capacity.
Sir Howard Davies, chairman of the Airports Commission, said Brexit underlined the need for a "clear strategic decision" in favour of Heathrow by ministers.
The Government will choose which scheme to back on Tuesday, ending more than a year of uncertainty since the Davies Commission came out in favour a third runway at Heathrow.
Writing in the Daily Telegraph, Sir Howard dismissed the idea of expanding both Heathrow and Gatwick - and hinted that Birmingham could be the next in line for a new runway once capacity in the South East had been expanded.
Transport Secretary Chris Grayling has acknowledged that any of the three options on the table - new runways at Heathrow or Gatwick, or extending an existing runway at Heathrow - would be controversial but would "open up new opportunities for Britain" as it adjusted to Brexit.
Sir Howard said: "The arguments for making a decision now, and for Heathrow, have strengthened in recent months.
"Overseas, the lack of a decision is seen as a symbol of Britain's inability to decide on its future as a trading nation. That may well be to overstate the case, but it is the way overseas businesses and governments view it.
"And the need for a clear strategic direction is more important since the referendum result. The rhetoric about becoming a European Singapore with a 'blue water' trading focus seems empty if we cannot connect to the new markets we wish to serve."
Sir Howard said Gatwick was largely a European short-haul airport while Heathrow had inbound passengers from around the world and "hugely more air freight, some 150 times as much as Gatwick".
Allowing new runways at both airports would be a mistake and "could mean neither is built" because of the risk of a legal challenge.
And once the high-speed HS2 rail line is built, "Birmingham might indeed be a more interesting option" than Gatwick.
Sir Howard blamed former prime minister David Cameron - who had given a "no ifs, no buts" guarantee that there would not be a third runway at Heathrow - for the delay in the decision since he presented his report, claiming he was an "immovable object".
Ahead of a meeting of the airports Cabinet sub-committee, Mr Grayling insisted all three options for expansion were still in play.
"Genuinely it's going to be a decision on Tuesday and it's a difficult one, because all three of these are well-crafted proposals and any one of them could bring benefits to the UK," he said.
Mrs May has moved to head off possible Cabinet resignations by giving ministers freedom to speak out against the Government's decision, with the possibility of Heathrow expansion fiercely opposed by Foreign Secretary Boris Johnson and Education Secretary Justine Greening.
"There will be challenge and opposition, whatever option we take," Mr Grayling told BBC One's Andrew Marr Show.
"The question here is that we have to, in my view, take a decision that is in the interest of our nation.
"What delivers us the best connectivity, the right approach for the future at a time when we want to grow international trade links, open up new opportunities for Britain.
"Of course there will be opposition, of course there will be challenge whatever we do."
Supporters of Heathrow have argued that a single hub airport is vital for connecting the UK to the world, but Mr Grayling indicated that new passenger jets made that less vital than in previous years "so there are genuinely competing arguments here".
Gatwick chief executive Stewart Wingate said attempts to expand Heathrow had repeatedly failed to make progress and insisted his airport represented the best option.
He said "Gatwick offers an end to this debate by providing a deliverable solution for balanced economic growth across the UK. With all the economic benefits at a fraction of the impacts, it is the obvious solution."