The toll on Bristol's Clifton Suspension Bridge may have to double to meet the rising cost of running and maintaining it, the trustees have said.
They said a review of the bridge's finances revealed that it cost £1m a year to run, which is about two thirds of its income.
This excludes the £8m cost of maintenance projects planned to take place over the next 10 years.
The toll for motor vehicles has been 50p since January 2007.
The body responsible for the bridge is a charitable trust which receives no financial support from central or local government, or from lottery funds.
They said investment income from stock markets had not materialised as the value of the investments had dropped in recent years.
The number of vehicle crossings has also dropped to 3.2 million in 2009 compared with 4 million a year between 1996 and 2004.
'Not yet finalised'
The trustees said it was essential that funds were ready to hand in case of emergency repairs, like the one prompted by a cracked suspension rod last year.
Tim Baines, clerk to the trustees, said: "While the trustees propose increasing the cash toll to £1 per crossing in order to 'balance the books', they stress that regular users of the bridge still will be able to benefit from a reduced rate, purchasing in advance bulk quantities of crossings stored on an electronic pass card which operates the barriers.
"Details of the proposed new rates for card users have not yet been finalised but, for our most regular customers who purchase either 400 or 1,000 crossings in advance, the increases are expected to be in line with inflation.
"While we can give no cast iron guarantees, because the bridge is now nearly 150 years old, we believe at present that we can raise the additional sums we require from visitors to Bristol and from non-regular users.
"For such occasional users we do not feel that, in this day and age, a £1 toll is excessive."
A proposal to increase the toll would have to be approved by the Department of Transport with the support of Bristol City Council and North Somerset Council.
There are no plans to re-introduce tolls for pedestrians or cyclists.