Transport for London (TfL) is considering charging motorists to park in its loading bays for the first time, BBC London has learned.
Currently drivers can park in bays on red routes - roads managed by TfL instead of councils - for 20 minutes.
But the organisation has said it is considering charging for the bays and admitted it was to raise revenue.
Campaigners warn the plans could breach common law which prevents parking from being used to make money.
In 1995, a High Court judge ruled in a case against Camden Council, saying it could not use parking to raise revenue.
The judge said authorities were allowed to profit from parking - but only when that was incidental to the wider goal of keeping streets free.
But when asked about TfL's plans, the organisation's transport commissioner Peter Hendy told the BBC: "If we need to change the regulations in order to be able to do so [charge for use of the bays] and it produces valuable income then that's what we will do."
Paul Pearson, who runs a parking fines appeal website, said: "It is very unfair to start charging motorists on red routes.
"To start charging them in this economic climate is unfair - and it may well be illegal."
Chris Stevens, who runs a paint shop in Holloway Road, Islington, north London, said his customers frequently used the free bays to load up purchases.
'Saving and efficiencies'
When asked what the impact on his shop would be, he said the charges would "close it down".
Mr Stevens continued: "Builders, who are my main customers, are not going to keep coming when they could go somewhere else."
A Transport for London spokeswoman said: "In responding to the government's Comprehensive Spending Review, the mayor outlined a number of steps that will be taken to deliver saving and efficiencies, enabling TfL to maintain investment in infrastructure.
"One of these proposed steps was to charge for parking on the Transport for London Road Network (Red Route). TfL is confident that it has the powers to charge for parking."
The plans were at a "very early stage", she added.