Entitled 'don't let our London Town die, come out and celebrate the city', the letter, written by St John co-founder Trevor Gulliver, has also been signed from a lobby of representatives from across the city, including those in the theatre, visual arts, live music, tourism and retail sectors.
Directed at Khan, the letter reads: "London Town, the driver for all that makes the greater city great, is at a critical point.
"It fails and the whole city fails. You have not been a source of support in this fight, indeed your agency was advising against travel and was promoting 'virtual events' until recently through (taxpayer money) London & Partners. These were hardly the acts of a champion for London.
"We are, like many, aware of the ongoing battles between City Hall and Westminster. However, now our concern is City Hall and, as some might suggest, its undemocratic policies guided by a cabal of people with particular agendas."
The letter asks the Mayor to do more to come out and promote the capital to the wider public.
"We need you as Mayor to get out on the stump alongside our famous, and many famously unemployed, citizens and shout out for London Town, the heart that has made London one of the great destinations of the world.
"The pandemic will end and the recession will deepen, start getting those messages out there, that we’ll be ready and waiting to entertain and delight.
"You can’t criticise Westminster when you adopt similar practices; legislate where necessary; apply the rules that will bring confidence and customers.
"Don’t hide behind spokespeople, who, quite frankly, do not answer direct questions."
The letter also demands the Mayor drop the congestion charge in a bid to encourage more people to visit the city; and push for greater confidence in people using public transport.
"Our transport system is arguably the best of the world’s great cities, that you seem to have no wish to use or celebrate this as a route to recovery seems a dereliction of responsibility.
"Again, leave the bunker; rise to the challenge; let the world and the suburbs and the home counties see that we are getting ready to welcome back all, and will impose the necessary practices to make the environment as safe as we can."
The letter, whose signatories also include Tom Kerridge, Richard Corrigan, Soren Jessen, Jonathan Downey, Des Gunewardena, Karam Sethi, Melanie Arnold and Margot Henderson, follows another written by Gulliver back in June, which asked the Government to provide more specific support for the capital’s businesses to avoid it becoming one of the biggest casualties of the Coronavirus pandemic.
"London Town lies at the heart of the greater city," concludes this latest missive.
"It dies, the rest follows. The core of our particular apple must be saved before it is too late. Please come out, open the gate and tend the orchard."