In a film about the campaign, the 88-year-old says he was pleased to pass on the information.
One document convinced the campaigners that the Skye Bridge company had no legal right to collect the tolls. The payments, introduced in October 1995, arose from the first Public Finance Initiative (PFI), introduced by Margaret Thatcher's Government, whereby the private sector would build public infrastructure.
Legislation had given the Secretary of State the right to collect tolls. To assign that right to a third party a toll order and a statutory licence called an assignation statement was required. However, the leaked Assignation Statement had no date or signature, leading to claims that it did not entitle anyone to collect tolls.
Mr Benn said: "If you are going to make a success of a campaign, first of all you have got to discover the truth and the truth is very often kept secret.
"The House of Commons sent me a very important document relating to the PFI but said that it was a secret document. They were allowed to give it to me because I was an MP but it wasn't to be released. This infuriated me even more because it confirmed my view that they were private arrangements at the back of this problem that had not been properly aired and brought out."
The document was presented in evidence in a High Court appeal, which the campaigners lost.
However, correspondence between the Crown Office and the Procurator Fiscal in Dingwall, which was released to The Herald under the Freedom of Information Act in 2005, showed prosecutors knew the legal paperwork was incomplete.
The allegations are made in a feature-length version of the film, The Bridge Rising, which will receive its world premiere at Glasgow Film Theatre on Sunday.