Gatwick Airport has begun distributing hundreds of thousands of leaflets across west London, warning of the noise impact that building a third runway at rival Heathrow would have.

Both airports are vying to be chosen by the next government for expansion, to increase flight capacity in south east England.

Gatwick - located on the border of Surrey and Sussex - would not reveal how much it plans to spend printing and delivering the 400,000 leaflets, in what could be its final PR offensive ahead of the General Election on May 7.

The leaflets began hitting doormats on Monday (April 13), with Uxbridge and South Ruislip due to receive 86,000 in total – more than any other constituency.

The leaflets claim that 683,000 people and 362 schools would be impacted by noise if a third runway were to be built at Heathrow, while only 36,000 people and 15 schools would be affected were Gatwick to expand.

Once again, the move has provoked a slanging match between the two airports, with both sides claiming the other has cherry-picked data.

Gatwick CEO Stewart Wingate said the information related to the proposed third runway option for expanding Heathrow, rather than the so-called Heathrow Hub option.

He said: “We felt it was quite important that we shone a light on this issue, in the west of London particularly and the area surrounding Heathrow, because we are not sure that this is fully understood by the residents, particularly those who would be newly impacted by noise.”

Gatwick claims the figures in its leaflets come from information given by both airports to the Airports Commission, which has been tasked with choosing which airport should expand.

The numbers, it says, relate to those who would experience an average noise level by 2050 of at least 55 decibels, a threshold recognised by the European Commission. Heathrow points out that it has offered to insulate homes within this contour.

Heathrow has also noted that the Airports Commission has said that were Heathrow to implement its plans for a third runway, which also involve various technological mitigations, fewer people would be affected by noise than are currently.

Conversely, the commission has said that building an extra runway at Gatwick would increase the number of residents impacted by noise, compared to current levels.

But Gatwick says the commission’s phrasing did not make clear that hundreds of thousands living near to Heathrow would no longer be impacted by noise in the future, even without any expansion, because of technological improvements and altered flight paths.

A Heathrow spokeswoman said: “Our plans deliver vast economic and social benefits, while reducing Heathrow’s negative impacts, and have been developed by listening to our communities.

“As a result, over 300,000 people could be taken out of the airport’s noise footprint with expansion.

“In addition, we are proposing a £700m schools and homes noise insulation scheme, which goes above and beyond government policy.”