Reliance has a confidentiality agreement with the prison service

The Scottish Information Commissioner carried out an investigation into the agreement, following an SNP complaint.

He has now expressed "dissatisfaction" with a legally-binding clause which allowed Reliance to stop full the publication of the contract.

Reliance will be paid �126m over seven years to provide court escort services.

The Scottish Information Commissioner is an independent public official, appointed by the Queen following nomination by the Scottish Parliament.

The role of commissioner Kevin Dunion is to make sure that people are aware of their right to access information under the Freedom of Information (Scotland) Act and then to enforce this right to make sure that people get the information they are entitled to.

'No public interest'

The Scottish National Party demanded publication of the Reliance contract after a series of high-profile mistakes which saw a number of prisoners and offenders go free.

The party lodged an appeal under the Code of Practice on Access to Scottish Executive Information.

Only an edited version of the contract was released with figures for cash penalties and other details missing.

The SNP complained and Mr Dunion launched an investigation and made his ruling public on Wednesday.

His report described the confidentiality arrangement as "extraordinarily unbalanced", giving a private firm a veto over the public interest.

The first minister should step in and demand that Reliance waives the confidentiality clause and allows the contract to be released in full Nicola Sturgeon SNP

But he conceded that the SPS was legally able to withhold the information.

His report said: "In my view, the SPS has not made a compelling case that applying the exemptions of the code of practice alone justify the withholding of the information.

"Nor, even if the exemptions did apply, have they demonstrated to me that the public interest lay in withholding rather than releasing the information.

"However, as the information requested is subject to a confidentiality agreement between SPS and Reliance, and given that the code of practice does not set aside statutory or other legal restrictions on disclosure, the SPS is entitled to withhold the information."

'Value for money'

The prison service said Reliance would not have signed without the agreement, driving up costs.

But the commissioner warned against transparency being traded off in this way.

He said: "My view is that public authorities must resist any suggestion from tenderers that the price of best value is to agree to withhold information which otherwise under freedom of information may be made available.

"Instead, companies must be made aware that if they expect to be successful in bidding for contracts paid for from the public purse, then they will operate under reasonable terms of scrutiny and openness provided for by the freedom of information legislation."

The SNP's Nicola Sturgeon said: "The first minister should now step in and demand that Reliance waives the confidentiality clause and allows the contract to be released in full.

"Only then will the public be able to judge if it is value for money."