A disastrous deal to build two new ferries was pushed through by SNP ministers despite officials warning them that it could be ruled unlawful, it has emerged.

       Previously secret correspondence shows that the contract was handed to Ferguson Marine, then owned by a pro-independence businessman, despite civil servants’ fears that other shipyards could challenge the decision in the courts.

       The risk emerged because SNP ministers insisted on handing the contract to the yard even after it reneged on a previously agreed funding guarantee designed to protect taxpayers if something went wrong.

       Officials warned that this represented a “considerable departure” from the advertised contract terms and this “may raise significant procurement risks”.

       The warning has only emerged because the Scottish Government failed to properly redact information in an email chain of previously “lost” documents, which it was claimed had been discovered on Wednesday and were published online.

       While the passages were blacked out, they could be read by highlighting the confidential text and copying it into another document.

       The email was hastily deleted after Douglas Ross, the Scottish Tory leader, revealed the blunder at First Minister’s Questions on Thursday.

       “In the latest incompetence from this SNP Government, [documents were] heavily redacted, lines blacked out,” Mr Ross said. “Except they’ve done such a poor job if you copy and paste, the information is revealed.

       “In one section it warned that the impact of a successful legal challenge could be high, in the worst case the contract could be declared ineffective.

       “In other words, going ahead with this contract was such a bad idea it could open up the Government to even more losses than they have already experienced. We all knew this was a bad deal, was it unlawful?”

       Nicola Sturgeon has repeatedly attempted to blame Derek Mackay, the disgraced former transport minister, for agreeing to the deal to build the two ferries.

       However, the documents also show that John Swinney, her current deputy, raised no objection to the move to hand the contract to Ferguson in 2015, despite the concerns of officials.

       The shipyard had been saved by the billionaire Jim McColl, one of the few high-profile business figures to back independence in the referendum as a favour to Alex Salmond before the independence vote.

       However, problems with the ferries meant the yard went bust again and had to be nationalised in 2019. The ferries, to serve island communities, had originally been due to be ready by 2018 at a cost of ￡97million.

       The cost has since snowballed to nearly ￡250million and the ferries are still not ready. The latest deadline for their completion is next year.

       Mr Swinney insisted on Thursday that he had merely signed off on the budget for the ferries contract in 2015 as finance secretary at the time, but that Mr Mackay had agreed the specifics. He denied giving the contract “the final nod”.

       However, documents released by the Scottish Government show that Mr Swinney was “aware of the background” of the deal and officials noted “the absence of banana skins” after he was consulted in October 2015. The deal was then sealed.

       Mr Ross added: “These new documents reveal that the person who signed off this deal was the Deputy First Minister John Swinney.

       “This contract had more banana skins than the monkey house at Edinburgh Zoo – but John Swinney couldn’t see any of them.”

       The previously redacted part of the emails state that any of the five shipyards which missed out on the contract could have raised a legal objection if they became aware of the terms handed to Ferguson.

       It stated: “The impact of a successful legal challenge could be high – in the worst case the contract could be declared ineffective – and a challenge could be brought at any time as the contract terms are not being made public.”

       Speaking at Holyrood, Ms Sturgeon repeated her claim that Mr Swinney had not taken the decision to sign off on the contract.

       She added: “He was simply briefed on the decision after he was taken. It is not unusual for finance secretaries to be briefed on all sorts of decisions that involve the spending of money. It does not mean that the finance secretary has taken the decision.”