A report into airport capacity across London has suggested using larger aircraft would avoid the need for a third runway at Heathrow.

The report to the Greater London Assembly transport committee, published this morning, said Heathrow was at 99 per cent capacity, while 12 per cent of runway slots are available at Gatwick, 47 per cent at Stansted and 51 per cent at Luton.

The report suggests Heathrow has spare terminal space and could serve 20 million more passengers per year if bigger aircraft were used but warns if a new airport was created in the Thames Estuary it could have a heavy impact on the use of Heathrow.

The information comes after The Mayor of London vowed to block a third runway at Heathrow during a rally against its expansion on Saturday.

Boris Johnson and MP Justine Greening gave speeches during the event in Barn Elms Playing Fields, Putney.

Visitors were invited to cast a vote during the event in a ballet box by Richmond Park Academy, while MP Zac Goldsmith and MP Edward Davey also showed up to show their support.

The rally follows a consultation which ended last week into night flights ran by the Department for Transport.

The existing night flights schedule for Heathrow comes to an end in October 2014 and the consultation into sleep disturbances will help plan the new system.

In January last year more than 1,000 people in Wandsworth complained of increased aircraft noise to the council since flight pattern trials began in November.

An alliance of more than 20 councils, known as the 2M group, said noise impacts of Heathrow’s flight pattern trials are being ignored by the airport operator.

The group believes a third runway would create extra demand from airlines for early morning arrivals from 6am, which would affect hundreds of people.

Mr Johnson: "Let me tell you, if they got their third runway, it would not be enough for that ‘moluck.’ They would say we need a fourth runway.

"They think they are secretly going to get that third runway but I can tell you, it’s not going to happen."

Ms Greening, Secretary of State for International Development, said: "There’s no room for any further expansion at Heathrow. I think whichever way you look at it there’s no more room."

A spokeswoman for Heathrow said: "We will examine the Civil Aviation Authority’s proposals for Heathrow’s Q6 regulatory settlement carefully over the coming weeks before responding fully.

To stay competitive with overseas hubs like Amsterdam, Paris, Frankfurt and Dubai, Heathrow has invested £11 billion over the last ten years in new facilities such as Terminal 5 and the new Terminal 2 and passengers say they notice the difference.

"Over the same period returns to shareholders have fallen well below the level anticipated by the regulator."

The results of the consultation are being analysed with the results due to be released later this year.

A second consultation on more detailed proposals will follow.