The Stop Heathrow Expansion group was launched ahead of the government's forthcoming consultation on a third runway. It said it wants to see better use of the existing runway.
The HACAN residents' group and ten MPs from the three main political parties have come together with the 2M group "to show the strength of opposition", according to 2M spokesman Steve Mayner.
"The group is not just a few angry residents. It represents communities as far away as Buckinghamshire, Merton and Kensington and Chelsea," he said.