The power to charge tolls was not mentioned in the original permit to operate the Skye Bridge, the BBC's Newsnight Scotland has learned.
Tolls were abolished last year but controversy continued over whether the bridge's private developer had the legal authority to charge motorists.
The permit was presented to the company by the government in 1995.
It allowed the bridge to be used on condition minor faults were remedied but made no reference to tolls.
Leading anti-tolls campaigner Robbie the Pict claimed the evidence, obtained under Freedom of Information laws, showed there was no lawful tolling regime.
"If the law requires a licence to charge tolls and no licence exists, surely the tolls must be illegal"
He wants convictions for non-payment to be overturned.
The campaigner said: "We've never been shown anything better than a draft version of the licence.
"It seems the civil service never saw the need to finalise this crucial bit of paperwork.
"Since the draft is unsigned and undated it has no legal status, so why was the bridge operator allowed to demand £33m in tolls and why do we still have criminal convictions for refusing to pay?"
'No action'
Dr Scott Veitch, from the Glasgow University School of law, said: "I think the important principle here is that where criminal convictions, many in this instance, are at stake, then the question of a procedural impropriety takes on a greater significance."
A Crown Office spokesperson said the legality of the tolling regime had been unsuccessfully challenged on several occasions.
No new information had come to light and no action would be taken regarding those convicted for non-payment of the tolls.
But Robbie the Pict said: "Basic logic dictates that if the law requires a licence to charge tolls and no licence exists, surely the tolls must be illegal."
The privately-built bridge was opened in October 1995.
It was Scotland's first private finance initiative (PFI), in which a public sector project is paid for and operated by a commercial company.
"Local hauliers ... are demanding their money back, which is in excess of £1m"
But campaigners claimed that an American bank paid for the construction in return for the right to charge the highest bridge tolls in Europe.
Approximately 130 protesters were convicted in court for refusing to pay the tolls and some were even imprisoned. The Scottish Executive abolished the controversial tolls after buying back the bridge from its private owners for £27m.
The old Highland Regional Council was the official agent of the scheme and the present Highland Council is now examining its position.
Councillor Drew Miller said: "I've written to the Highland Council convener asking her to support the calls by the local hauliers who are demanding their money back, which is in excess of £1m.
"Over and above that the council is due back about £500,000, that's a considerable amount of money, you could build half a school for that.
"The convener has asked the head of legal services to look at the documents which are unsigned."
Transport Minister Tavish Scott declined to comment on whether there had been a sound legal document licensing a tolling regime.
But Emeritus Professor Ian Willock, from the Dundee University School of Law, said: "It seems to me unlikely that these civil faults would be enough to undermine the convictions, even less the return of the fines."
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