Johnson has joined forces with the 2M group of local authorities fighting the plans. They will analyse the government's decision, which is expected next month. The councils argue that an increase in flights to 700,000 a year would breach EU laws on air pollution due to come into force in 2010.
EU environment commissioner Stavros Dimas has vowed to take action if limits are breached. Environment secretary Hilary Benn confirmed that the government will seek permission to postpone compliance until 2015.
"Expansion will only go ahead if strict environmental conditions on air quality and noise can be met," a DfT spokesman said.
Meanwhile, the 2M group presented its case for a high-speed rail link to the Scottish Government's inquiry on the subject this week. The network would connect London to Glasgow, Edinburgh and Aberdeen and to Leeds, Liverpool and Manchester.