Thousands of Transport for London (TfL) staff are to be furloughed amid "massive financial challenges".
TfL's income has been badly affected during the pandemic, and about 7,000 employees will be put on the government's furlough scheme.
London's transport commissioner Mike Brown said fares, which are TfL's main revenue, had plunged by 90%.
The Mayor of London warned transport "will not immediately return to normal" when lockdown measures are relaxed.
Staff will initially be furloughed for three weeks from Monday and TfL said it "will pay the remainder of salaries of all furloughed employees and continue to pay pension contributions, to ensure people are supported".
Currently a limited service across the capital is in place to allow "essential travel" for key workers.
Bus passengers are no longer allowed to board using the front door and do not need to tap in with an Oyster card.
Since London entered lockdown on 23 March, Tube journeys have fallen by 95% and bus journeys by 85%.
Sadiq Khan said he wanted "to be honest and upfront" about the impact on public transport in the capital and its future.
"There will be no quick return to business as usual," he said.
"Covid-19 has caused massive financial challenges for TfL and every transport provider across the UK.
"As the only transport authority of any major city in western Europe which doesn't receive a grant for day-to-day operations, the challenge for TfL is very acute."
Mr Khan said TfL, which has 28,000 employees, would be in talks with the government and various trade unions about putting staff on to the furlough scheme.
"TfL is urgently working through how it can get Londoners to and from work while social distancing rules remain in place, as is widely expected to be the case," he added.
Mr Brown said he "hoped for an urgent agreement" between TfL and the government.
The Department for Transport previously said that "regular discussions" were taking place with London's mayor and transport authority about financial problems caused by the lockdown, and they "will continue to do so".
By BBC London's Transport correspondent Tom Edwards
It has been called the most serious financial challenge ever faced by the capital's transport agency.
Using just simple maths TfL is in trouble - you can't run a transport agency without fare revenues. Now it has set out what it will do and it involves furloughing a quarter of its staff.
Also interesting is how the future of London will look when the lockdown ends. If social distancing is in place buses and the Tube will only be able to carry a sixth of passengers.
That means a huge campaign of "travel management" to get passengers to phase journeys and shift the rush hour - the like of which we haven't seen since the Olympics.
But, big infrastructure projects also now hang in the balance. TfL's future is entirely dependent on a government bailout - which will have to be big.
At present it costs £600m a month to run the system - or there will be cuts to services.