London Mayor Boris Johnson says Britain could lose tens of billions of pounds in tourism unless a huge airport is built in the Thames Estuary.

In a speech on Monday, the top Tory 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 government must now “begin serious plans for the multi-runway solution that can keep London and our great nation in the premier league of the global economy”.

Medway and Swale councils have 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 harm 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 estuary off Sheppey.

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

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

He added: “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.”

The Stop Estuary Airport campaign, which is run by Medway council and Kent County Council and supported by the RSPB, is calling on opponents of the idea to email Mr Johnson and the Transport Secretary Justine Greening.

The message, which can be sent from the campaign’s website, reads: “Dear Boris, I am writing to say that I do not feel that there is a need for your pipe dream of an airport scheme on or near the Thames Estuary and believe that you should drop this pie in the sky idea with immediate effect.”

To send the email, visit www.stopestuaryairport.co.uk/email-boris-johnson/