Gatwick says the £2.2bn project would not need government money, would be 100% privately funded, and could be complete by the end of the decade.
Today's update comes after the chancellor said last month the government also supported a third runway at Heathrow as part of its wider effort to bolster UK economic growth.
Greenpeace UK's policy director, Doug Parr, said of the process ahead: "By approving Gatwick's expansion the government will hang a millstone the size of a 747 around the country's neck.
However, the formal planning process is still to take place and the runway would be unlikely to open until the end of the decade, assuming any legal challenges were swiftly overcome.
The airport is already the second busiest in the UK, and the busiest single runway airport in Europe.
