The Stop Bristol Airport Expansion group said granting a certificate of lawfulness for the project has opened a back door that will allow the airport to expand by stealth. The decision was made at a council meeting last week.
Opponents argued that the two-storey structure is more than four times the length of the existing terminal and will enable annual passenger numbers to rise from 6.5 million to 7.5 million, thus increasing capacity without planning permission.
The airport's director of environment and planning Alan Davies maintained that the walkway will reduce coach transfer to and from flights, cutting emissions.
The campaign group also pledged its opposition to the airport's proposed planning application to take passenger numbers to ten million each year by 2016. The airport is expected to submit its major development plan in March.