An influential Tory MP is ramping up pressure on the Government to reveal if it made any financial promises to Japanese carmaker Nissan.
In October, Nissan said it was extending production of two new vehicles in Sunderland amid rising speculation that it could ditch the UK following the country's decision to quit the European Union.
Andrew Tyrie, chairman of the Treasury Committee, has written to the head of the National Audit Office, Sir Amyas Morse, to ascertain if any contingent liabilities have been created in relation to assurances provided to Nissan.
Mr Tyrie said: "The Chancellor has not answered the crucial question: whether an assurance was given to Nissan which could constitute a contingent liability, or support consistent with the provisions of the Industrial Development Act 1982.
"So I have written to Sir Amyas, asking him to examine whether contingent liabilities have arisen as a result of Government commitments, and/or assurances made to Nissan."
The move secured 7,000 jobs in the Brexit-backing city, but prompted a volley of questions over whether a so-called "sweetheart deal" between the carmaker and the Government had been struck.
Mr Tyrie's inquiry comes a day after Chancellor Philip Hammond wrote his own letter to the Committee, saying any costs arising from the assurances would be small enough to be covered within the Department for Business's existing departmental expenditure limits (DELs).
Last week the Office for Budget Responsibility revealed that the Treasury had point blank refused to disclose if it had made any financial promises to Nissan, bemoaning: "Unfortunately, the Treasury declined to address the substance of our question."