A public consultation is under way
A congestion charge may not be introduced in Greater Manchester even if its residents vote for it, the transport secretary has said.
A public referendum is to be held on plans for the £318m scheme, which is part of a £2.8bn package of public transport improvements.
But Ruth Kelly said ministers would still need to assess its value for money before giving approval.
She told the BBC the December public vote was "not the final say".
The Bolton West MP confirmed the government's provisional approval for Greater Manchester's bid for funding from the Transport Innovation Fund (TIF) in June.
Under the bid the region would get £2.8bn to create a "world-class public transport system", subject to the introduction of a charging scheme in 2013.
If the charge gets the green light, motorists would be charged for crossing the M60 and a second ring around the city centre at peak times.
A massive publicity campaign is being waged by groups on both sides of the debate ahead of the referendum in December.
But speaking at the Labour Party conference on Tuesday, Ms Kelly reiterated that it would be the government that made the final decision.
"There's a massive chance for Greater Manchester but obviously the case has to be made," she said.
"Obviously people need to be consulted and we only want to work with the willing.
"So even if there were a yes vote in the referendum the government would still need to decide whether or not the financial case stacked up."
'Inefficient and expensive'
However, Greater Manchester Momentum Group (GMMG), which is made up of local businesses and organisations opposed to the scheme, said it was time to "go back to the drawing board".
A spokesman said: "We strongly challenge the suggestion that this is a massive opportunity for Greater Manchester, current proposals will be both damaging to our economy and ineffective in tackling congestion.
"Before carrying out a costly referendum we should go back to the drawing board and explore real alternatives that could better meet our long-term transport needs."
A spokesman for the National Alliance Against Tolls said it believed the scheme was "very unpopular".
"Roads users can give them the answer now - road tolls are the most inefficient and expensive way of financing any scheme," he said.
A public consultation is currently taking place which will define the final form - and finer details - of the region's TIF application.
The referendum will decide whether that application for TIF funding is actually submitted to the government.