Spending in central London is set to fall by £10.9bn as a result of Covid-19, according to a report.
The money brought in by foreign tourists is set to drop £7.4bn, according to the mayor's office, while domestic tourism will fall by £3.5bn.
London mayor Sadiq Khan said the data showed government bailouts were only a "sticking plaster solution".
The government has announced changes to its Job Support Scheme, accepting some sectors "needed more support".
Under the new scheme, employers will pay less and staff can work fewer hours before they qualify.
But Mr Khan said the city's tourism sector needed more specific support.
His office said commuters would normally spend £1.9bn travelling into central London but this had also been lost.
The analysis has been carried out by the mayor's office, based on a short-term forecast by VisitBritain which said it expected there to be a 77% drop in foreign tourism and a fall of 48% in domestic tourism compared to 2019's figures.
VisitBritain said: "Forecasting at this time is difficult, given the fast-moving situation and the unique circumstances.
"This central scenario assumes that a vaccine will not be widely available during 2020. It assumes that the UK will not go back to the April-June situation of a full national lockdown and near-zero international travel, although it is consistent with local lockdowns being imposed," it added.
The tourism industry accounts for one in seven jobs in the capital and contributes almost 12% of London's GDP.
Mr Khan said its collapse meant any hospitality, retail and leisure businesses would be left without hope of returning to normal for "many months to come".
"This analysis shows the eye-watering drop in the money spent by visitors in the economic and cultural heart of the capital - which is far greater than even the impact of fewer commuters travelling into the centre of the city each day.
"These sectors won't be able to sustain pre-pandemic levels of employment until tourists return in significant numbers, so government must act swiftly to prevent widespread job losses and the financial hardship this will bring for Londoners, many of whom are already struggling to make ends meet."
Baroness Wheatcroft, who chairs the Association of Leading Visitor Attractions, said: "Everyone in London knows someone who works in tourism and they know how decimated the industry is.
"We need specific help from the government for our hospitality and tourism sectors to keep businesses afloat, keep people in their jobs and to market London to the UK domestic market, and to the world, so we can welcome back our inbound tourists when they are ready to return."
The government said "these are difficult times for regions around the country," adding the Job Support Scheme and other measures will help support those affected as part of its "comprehensive package of support aimed at one thing: protecting jobs".