Boris Johnson says Britain faces a period of "economic stagnation" unless a huge airport is built in the Thames Estuary.

In a speech today, the London mayor said the airport would create tens of thousands of jobs – many in Medway.

He warned that without it, more than £62 billion of tourism could be lost as airports in the south east reach capacity within 20 years.

Mr Johnson said the governemnt must now "begin serious plans for the multi-runmway solution that can keep London and our great nation in the premier league of the global economy."

Medway Council has branded plans for an airport in the Thames Estuary "pie in the sky" and vowed to oppose them along with the RSPB, which says they will damage bird life.

Two estuary airport plans have been proposed – one on the Isle of Grain by the renowned architect Lord Foster and the other, dubbed Boris Island, in the middle of the Thames Estuary between Kent and Essex.

National newspaper reports suggest chancellor George Osborne supported the idea of a new airport, but his spokesman declined to comment.

Shadow chancellor Ed Balls did not rule out his backing for building a new airport in the estuary on a visit to Chatham this week.

He added: "People want to fly and I think airport expansion is a very important part of what we need to do across the south east, but it has to be done in a way that listens to local residents."