Heathrow has begun publicly handing over its crown to Dubai as the world's busiest international airport, saying it can no longer compete.
The admission follows two months in which the Middle East hub trounced Heathrow's passenger figures, the first time this has happened.
Heathrow chief executive Colin Matthews said Dubai's achievements "shows that the UK will soon no longer have the world's number one airport for international passenger traffic".
Heathrow hopes Dubai's passenger figures for January and February will sway The Davies Commission, which is still deciding whether to recommend that Heathrow gets a third runway.
Matthews, who leaves in June, added: "We want Britain to continue to compete globally against the best hub airports in the world but without a third runway. Heathrow's comparative decline will make the whole of the UK a less attractive place to do business."
During March, Heathrow saw 5.8 million passengers through its doors, down 2.8 per cent year on year, because of the late Easter this year.
Figures also reflect the growth of new destinations and the use of bigger aircraft. Passenger numbers to Mexico were up 19 per cent year on year, China up nearly 12 per cent and Turkey by 7 per cent. The average number of seats per aircraft rose almost 2 per cent.
Gary Noakes