Any plans to expand London's Night Tube service would be met with "big financial challenges", the capital's "Night Czar" has said.
The overnight service covers five of the 11 Underground lines, as well as the Overground's East London line.
Amy Lamé, appointed by the London mayor to ensure the capital thrives as a 24-hour city, said Transport for London's tight finances could delay expansion.
Sadiq Khan previously outlined proposals to grow the Night Tube.
His spokeswoman said those plans, from his 2018 Transport Strategy, had not anticipated the pandemic and subsequent funding crisis.
She said they were no longer an accurate depiction of his priorities.
Ms Lamé, who has been in the role since 2016, said any further expansion of the Night Tube would be dependent on the completion of the Four Lines Modernisation programme, a project improving the Circle, District, Hammersmith & City and Metropolitan lines.
The scheme, which was due to be finished at the end of the year, was delayed by the pandemic and a revised timetable has not yet been announced.
Ms Lamé added: "It's not a secret that TfL has had some financial challenges, due to the pandemic and due to the way central government has been treating TfL.
"I'd say for the future, nothing is off the table, but I think there are big financial challenges,"
By Tom Edwards, BBC London transport correspondent
It seems like the Night Tube for the moment has expanded as far as it can. It reads like this is partly financial, partly an engineering issue but also political priorities have changed.
TfL has financial constraints, which means it can't expand services too much – it is making cuts in other areas. And the Night Tube never made that much money.
And even if it could expand the Night Tube, it seems like there are limitations technically on the remaining lines, which are being upgraded.
Perhaps more likely is the Elizabeth line. I have heard rumours of the Elizabeth line running through the night – although you'd think anything like that would be after its final stage of completion in May.
The Night Tube was one of the first services that was suspended at the start of the pandemic, and was reopened several months after the last lockdown ended, with the Northern and Piccadilly Lines reopened in July 2022.
A spokeswoman at the Department for Transport says it is for the mayor and Transport for London (TfL) to ensure the London transport body is able to sustain itself financially.
She added in August last year, the government agreed a longer-term funding settlement to provide TfL with just under £1.2bn of grant funding until the end of March 2024, or until TfL becomes financially sustainable, if sooner.
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