A Department for Transport spokeswoman explained that the study on Attitudes to Noise from Aviation Sources in England (ANASE), which began in 2001, is being peer-reviewed at present. But she could not say when results would be published.
Campaigners have accused ministers of withholding the report because it will undermine the case for expansion of the west London airport, including a third runway.
A draft copy obtained by The Times said the methods used to measure noise are outdated and people are less tolerant of aircraft noise than 25 years ago, when the previous study was made.
The ANASE report suggests the accepted maximum aircraft noise of 57 decibels is too high and should be revised to 50 decibels.
Campaign group HACAN Clear Skies claimed that the study increases the estimated number of people affected by aircraft noise at Heathrow to 2.5 million from previous figures of between 250,000 and 350,000.
Aviation minister Jim Fitzpatrick wrote to the 2M group of local authorities around Heathrow last month stating that public consultation on airport expansion can begin later this year, before the study is released.
However, 2M authorities spokesman and London Borough of Wandsworth leader Edward Lister said this consultation will be unbalanced without the study.