Gatwick Airport has accused the campaign group Back Heathrow of misrepresenting itself as a community organisation and winning support by scaremongering.
He said Back Heathrow 'wears the mask of a community group when they are simply a campaign arm of Heathrow, set up and funded by the airport.'
Mr McNulty went on to accuse Back Heathrow of 'scaremongering' by giving the false impression in a survey distributed to 750,000 people that without expansion the airport would be forced to close.
He added: “I believe that the commission, and everybody else, should view the statements and outputs from the Back Heathrow campaign with strong scepticism.”
Back Heathrow received initial funding from Heathrow when it launched, which it announced at the time.
Responding to Sir McNulty's letter, the group's campaign coordinator, Rob Gray, said: Back Heathrow is proud to be supported and financed by local businesses and residents as well as Heathrow Airport and Heathrow Hub.
"The uncomfortable truth for Gatwick and those opposing expansion is that the majority of local people in west London support a bigger, better and quieter Heathrow which will provide jobs and economic security for the long term."
Back Heathrow claims to have 50,000 supporters and says that number is growing.
Giving evidence today, Mr Gray described the organisation as 'a local group of residents, businesses and community groups campaigning for Heathrow and for the communities that rely upon Heathrow flourishing'.
He added: “For people whose communities, livelihoods and family prospects depend on Heathrow thriving in the future, the stakes couldn’t be higher.
“We are delighted to have the support of both Heathrow and Heathrow Hub. Frankly, I would be disappointed if we didn’t.
“Yet we only really exist because of our thousands of supporters. They are our core and our strength. They are the reason we are here today.”
The commission has been tasked with deciding which would be preferable for increasing flight capacity in south-east England: building a third runway at Heathrow, extending one of its existing runways, or building another at Gatwick.
Proponants for Heathrow's expansion say it would boost the economy and create jobs, while opponents say it would create pollution and destroy communities.