Transport for London “remains vulnerable” despite ministers handing the authority a £1.8bn bailout last weekend.
Analysts from Moody’s estimate TfL will rack up losses of more than £3bn this financial year.
Last weekend the organisation agreed a six-month funding package from Department for Transport amid fears that public transport in the capital could grind to a halt.
However, analysts from the ratings agency underline the importance of “a longer-term solution to TfL's financial sustainability”.
TfL has historically raised money on the public debt markets.
Meanwhile, Moody's points out that revised budgets estimate a £2.9bn deficit in the year to March 2022.
“Although we assume some efficiencies will be implemented in this year, the majority of this shortfall would need to be funded through grant, in the absence of material additional funding sources.
“TfL remains vulnerable to the economic slowdown as a result of the coronavirus shock as well as restrictions implemented to manage virus transmission.”
TfL’s funding crisis boiled over into a war of words between London mayor Sadiq Khan, the transport authority’s chairman, and Boris Johnson.
The prime minister accused Mr Khan of “bankrupting” TfL - an allegation that the mayor rejected.
The mayor had to agree to a number of concessions to secure central government funding.
A spokesman for TfL said: “The pandemic has also meant our fare and other revenue has been significantly impacted and will take time to recover. Our recent funding agreement with the government allows us to help London through this next phase of the pandemic.
“We have always been clear that further financial support will be required next financial year and continue to work with the mayor and the government on securing a long term, stable and sustainable funding deal, while doing what we can to minimise costs.”