LONDON (Reuters) - Britain's Heathrow Airport said on Thursday it could use new technology to add 5 percent more flights, as it tries to gain support ahead of a long-awaited government decision on where to increase London's airport capacity.

After a decade of reviews, U-turns and environmental protests, the government is expected to rule in mid-October on whether to build a third runway at Heathrow, Europe's busiest airport which is almost full, or its smaller London rival Gatwick.

In the past, concerns about air and noise pollution and local opposition have made expansion at front-runner Heathrow difficult to deliver politically.

But if it is selected, Prime Minister Theresa May would win parliament's backing for expanding the west London hub, the Financial Times reported.

Heathrow said it could add 5 percent more flights from 2021 by reducing gaps between aircraft movements, thus expanding capacity before any new runway comes into operation in 2025.

It said the additional flights, derived from new air traffic technology called Enhanced Time Based Separation, would be allocated to routes which would help support Britain's export market by connecting UK cities, and markets in Japan, India and China.

Heathrow, Britain's biggest port which handles a third of the country's non-EU exports, says that Britain's vote to leave the EU makes it more important that it secures approval to expand, as it can build more routes with trading nations.

For its part, Gatwick says it can build a new runway at a lower cost and with less environmental impact than Heathrow.

Trying to win support for its case, Gatwick said this week it was carrying more passengers and serving more long-haul destinations than forecast by the Airports Commission, a key independent inquiry into London's runways which in 2015 came out in favor of Heathrow expansion.

There are two options for Heathrow expansion on the table, building a new runway or expanding one of the current pair.

(Reporting by Sarah Young; editing by Stephen Addison)