The Government has refused to tell its own economic watchdog whether it made any financial promises to persuade Nissan to stay on Wearside.
The Japanese car giant announced last month that it is to build two new models - a third version of the Qashqai and a new X-Trail - at its Sunderland plant.
Nissan had warned it might have to reconsider investment in the plant in the light of the UK vote to leave the EU.
The production announcement followed a meeting at Downing Street between Prime Minister Theresa May and Nissan global boss Carlos Ghosn.
Both the company and the Government have refused to reveal exactly what reassurances were given to the company.
Ministers have strongly denied suggestions they offered the firm a sweetheart deal but have refused to publish a letter of reassurance sent to Mr Ghosn.
Now it has emerged the government has even refused to release details of the discussions to its own Office for Budget Responsibility (OBR)
The OBR is responsible for providing independent and authoritative analysis of the UK’s public finances.
Its economic outlook report, released alongside the Chancellor's Autumn Statement today, reveals the watchdog quizzed the Treasury on whether there were any financial implications from the talks with Nissan "and the Treasury declined to say."
In compiling its forecasts, the OBR also asked to which department's accounts the Government expects any liabilities to be reported.
However, the OBR said: "Unfortunately, the Treasury declined to address the substance of our question, telling us instead that 'There is a standard process for departments to report to Parliament as and when they incur contingent liabilities. Any commitments incurring costs will be managed within existing overall Departmental Expenditure Limits totals.'"
The Government has insisted that there are no liabilities relating to its assurances "as it currently stands", but refused to be drawn on whether this will change once Britain leaves the European Union.