The Welsh Government has indicated for the first time it would only have been prepared to guarantee around a quarter of the cost of the proposed Circuit of Wales racetrack in Ebbw Vale.
Ministers rejected the £433m project last summer after being asked to underwrite around half of the figure.
Top civil servant Dame Shan Morgan told AMs a "radical change" to the backers' plan would not have been possible.
The scheme aimed to create up to 6,000 jobs in an unemployment blackspot.
Instead plans are now underway for a £100m automotive business park in the area to be developed by the Welsh Government.
Circuit backers the Heads of the Valleys Development Company (HoVDC) had wanted the Welsh Government to underwrite 80% of the cost, revised down to about 50% when ministers asked for a rethink.
Alongside the circuit itself, which would have hosted the British leg of the MotoGP for at least five years, there were also plans to attract BMX and mountain biking, indoor skiing and concerts.
The issue of the guarantee meant the Welsh Government would have received £99m over 33 years in exchange for underwriting the project once it was open for business.
But, in the event of it failing, the public purse would have had to pay back £210m over time, while backers Aviva would keep the circuit and the infrastructure.
Questioned by the assembly's public accounts committee, the Welsh Government's permanent secretary Dame Shan Morgan said the final analysis found that "any solution would only be identified if the Welsh Government guarantee were to be reduced by at least 50%".
She went on to say: "Officials concluded that something that would amount to a very radical change to the finance structure would not be possible."
Dame Shan said that new information had been constantly coming into the Welsh Government during the course of six years as the project underwent significant changes.
She said the developers had been given a lot of time to make the project work and had been kept aware of the issues.
The cabinet decided to reject the proposal in June 2017, Dame Shan said, because of a mix of factors, including failing to meet the government's objectives and also because developers could have walked away from the automotive business park element which was due to have created the bulk of the new jobs.
She added: "The assessment was also made that the benefits that were claimed for the project and in particular the number of jobs created by the initial circuit element of the proposal were, let us say, uncertain at best."
Her response prompted criticism from Plaid Cymru AM Adam Price and UKIP Wales leader Neil Hamilton, who accused the Welsh Government of "moving the goalposts" for the company in terms of financial support.