In the letter, Johnson said: "We are delighted to support the Sustainable Events Summit in its mission to share knowledge and learning from global brands and leaders from the events industry with the wider live marketing and communication sectors.
"The event will build on the legacy of the 2012 Olympic and Paralympic Games to showcase best practice and provide knowledge and expertise to the UK event industry. I wish you a successful summit and look forward to hearing about the resulting actions and outcomes for a more sustainable industry."
The letter from City Hall comes on the back of support from other government figures.
Minister for Sport and Tourism, Hugh Robertson, said: "Delivering a sustainable London 2012 Games was a key priority right from the outset and they have been hailed as the greenest ever Games by the Independent Commission for a Sustainable London 2012. The Sustainable Events Summit will be a chance for a wide range of global brands to come together, share knowledge and best practice and make sure that sustainability continues to be an integral part of staging future events."
The full agenda has been finalised and now includes speaker information.
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