However, the campaign group has complained that the ruling is "confusing" and accused the Advertising Standards Authority of being "on wafer-thin ice".
It is the latest episode in a long-running fight between the UK's two largest airports, during which Back Heathrow has been accused in some media of 'astroturfing' – using corporate money to give the incorrect appearance of grassroots support.
The ASA ruling, published today, relates to a regional press ad for Back Heathrow from October 2015, headlined 'Rallying for the runway', which had text stating: "Don't believe the hype. Most people living in communities near Heathrow Airport support its expansion."
A footnote to the ad said: "The latest independent polling shows 60 per cent of local residents expressing an opinion support expansion." It also said this was based on a poll of 12,000 residents from 12 local constituencies.
Five complainants challenged whether the claim "most people living in communities near Heathrow Airport support its expansion" was misleading and could be substantiated.
In its response, Back Heathrow said that it had ensured that it stipulated the constituencies following the detail of a September 2015 ASA ruling about a separate case involving Heathrow Airport itself, and said that of the 12,000 residents questioned in the research, 50 per cent had expressed support for Heathrow's expansion, and 33 per cent had opposed it, from which it arrived at the 60 per cent figure. It also cited another survey in which it said that 53 per cent of residents were pro-expansion.
"The ASA noted that headline claim stated 'Most people living in communities near Heathrow Airport support its expansion', which we considered most consumers were likely to understand to mean that a clear majority of those surveyed in the poll (the original sample) were in support of expansion," the ASA said in its judgement, concluding that the ad had therefore breached the Committee of Advertising Practice's Code, and must not be repeated. The ASA also said the ad had made it appear that the constituencies in question were those closest to Heathrow, despite this not being the case.
Rob Gray, Back Heathrow campaign director, said: "The ruling is confusing because even by the ASA's own definition of residents living near to Heathrow, the evidence clearly shows a majority of support for the airport's expansion. Nevertheless, we are pleased that the ASA acknowledged that more local people support a new runway at Heathrow than oppose it.
"In the recent Airports Commission public consultation, 82 per cent of the 70,591 responses were in favour of Heathrow expansion. Most of these were local residents so we think the ASA is on wafer-thin ice with this ruling. In the 10 constituencies defined as 'local' by the ASA, there is a 53 per cent majority in favour of Heathrow expansion."
Convened in 2012, the Airports Commission closed in July 2015 as it published a final report that "unanimously concluded" a third runway at Heathrow was the best option for London and the UK economy. However, the bitter battle between it and Gatwick continues after the Government said it would not act on the report, a decision connected to next month's London mayoral election, with the Conservatives' candidate Zac Goldsmith opposing growth of Heathrow.