The soaring costs come as the Scottish Government continues to work behind the scenes on the impact of the collapse of Greensill Capital on GFG Alliance, the owners of the Lochaber smelter.
Critics labelled the figures “staggering” and described it as a “disastrous indictment of SNP industrial policy”.
In 2016, ministers agreed a £586m guarantee with Liberty Steel, a subsidiary of GFG Alliance.
The agreement saw the Government guarantee 25 years of power purchases by Mr Gupta’s company from another business owned by his father, with the guarantee funding the purchase of the aluminium smelter.
The exposure to the taxpayer due to the deal was valued at the time as £586m, with the Government stating the remaining exposure sits at £284m with 20 years of the guarantee left to run.
Ministers said recent accounts valued the assets in Fort William at £438m.
Questions have also been raised about whether the deal may have breached state aid rules.
Deloitte has been working for the Scottish Government for five years on the Lochaber deal, providing quarterly updates on the guarantee.
This includes examining whether Liberty Steel has been adhering to the terms of the deal and advising the Government on how to mitigate the collapse of the company’s main funders, Greensill.
The cost of this work has skyrocketed in the past year, rising to more than £800,000 for the 2021 financial year, figures only released following a Freedom of Information request by The Scotsman.
This is a significant rise compared to previous years.
In 2017 Deloitte were paid £112,000, rising to £180,000 in 2018, £350,000 in 2019 before dropping to £218,000 in 2020.
Scottish Labour's finance spokesperson Daniel Johnson said this was taxpayer money being spent due to an “SNP secured dodgy deal”.
He said: “The numbers involved are staggering and the public are right to ask questions as to why this contract was needed and why the costs have escalated so much.
“After the ferry fiasco, this is another deal in a litany of dubious contracts signed by the SNP.”
Scottish Liberal Democrat economy spokesperson Willie Rennie added: “If only the Lochaber deal had proven as lucrative for the Scottish Government or the highland economy as it has for Deloitte.
"Ministers struck a deal that they said would provide 2,000 jobs for the town. Those jobs have never been delivered, but international business consultants have raked in hundreds of thousands in fees.
"This is a disastrous indictment of SNP industrial policy."
Liz Smith, the Scottish Conservative finance spokesperson, said the sums were “very substantial” and were handed over with “no clear accounting or transparency”.
She said: “It is typical of the way that SNP tries to ensure its financial dealings evade scrutiny. They should bear in mind that this is taxpayers’ money, not their private purse.
“The public need to be clear that the rules are being followed, and the SNP must be open about exactly why so much is being handed to a single firm.”
A Scottish Government spokesperson said: “The Scottish Government contracts input from specialist advisers such as Deloitte to protect its interests. Since 2017 Deloitte has been retained as commercial advisers on matters relating to the Lochaber guarantee. This includes regular review of business plans and financial monitoring reports prepared by the business plus advice on contingency scenarios.
“The Scottish Government’s intervention to support the Lochaber aluminium smelter has preserved strategic industrial capacity and supported the livelihoods of hundreds of people. The Scottish Government receives a commercial fee in respect of the Lochaber Guarantee and Guarantee fee payments are up-to-date. There has been no call on the Lochaber guarantee and therefore no public expenditure has been incurred.”