The number of flights at Heathrow will increase far sooner than the Government has previously admitted under plans to squeeze in more take-offs and landings on the existing runways.

BAA, the Spanish-owned airport company, plans to increase the number of flights over several years in the hope that those living under the flight paths will not notice that noise is spreading throughout the day.

A third runway, to open in 2020, is expected to be approved by ministers before Christmas. However, they will allow expansion to begin as early as 2010, with at least another 60,000 flights a year using the existing two runways by 2015.

BAA admitted yesterday that residents could be exposed to much longer periods of aircraft noise from 2010. It is also preparing a planning application to increase the maximum number of flights at Heathrow from 480,000 to 540,000.

The Government's consultation document, Adding Capacity at Heathrow, implied that the cap on flight numbers would not be lifted until 2015. However, the Department for Transport said: “Additional capacity could potentially be added under mixed mode before 2015.”

Mixed mode means effectively treating each runway as a separate airport. As soon as one aircraft lands, another will turn on to the same runway and prepare to take-off.

At present one of Heathrow's runways is used for landings and the other for take-offs, with their roles switching each day at about 3pm. This alternation gives people living under flight paths half a day's respite from the worst noise.

A BAA spokesman said: “The number of flights could increase above 480,000 before 2015, but only subject to a planning inquiry and meeting the Government's strict environmental conditions.”

The 2M Group, an alliance of local authorities close to Heathrow and Stansted, has calculated that mixed mode could add 170,000 flights a year at Heathrow. The DfT will propose that mixed mode should be limited initially to peak flight times, possibly 6am-10am and 9pm-11pm, with runway alternation at other times.

Colin Matthews, BAA's chief executive, wrote to every MP yesterday, proposing that the Government should appoint an independent assessor to decide whether Heathrow was meeting legal limits on air pollution and noise. Expansion would be postponed if the assessor decided that limits would be breached.

Opponents of expansion at Heathrow, however, said that BAA could not be trusted. In 1999 BAA said in a letter to residents: “We do not want, nor shall we seek, an additional runway.” Five years later BAA published a proposal for a third runway.

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