The Treasury has refused to tell the Office for Budget Responsibility if there was any cost attached to Nissan’s decision to stay in the UK, raising further questions about assurances given to the Japanese carmaker.

The OBR said it asked the Treasury before the autumn statement if there were any potential costs that could upset its economic forecast.

It said: “On this occasion we asked specifically whether any contingent liabilities had been created in respect of assurances provided to Nissan and the Treasury declined to say.”

Nissan said last month it would build two new models at its plant in Sunderland after suggesting it would leave the UK if it faced onerous tariffs caused by Brexit.

The government said it had reassured Nissan about its commitment to getting a good deal for the wider car industry and that it made no financial commitments.

But questions have lingered over what Nissan was promised. Andrew Tyrie, chairman of the Treasury committee, asked the chancellor for more information by 18 November but has received no reply.

Jolyon Maugham, a barrister, said the Treasury might have broken the law by failing to provide the OBR with information.

He said: “The OBR has mentioned it twice in the fiscal outlook, suggesting they regard it as being a matter of real public importance.”

A spokeswoman for the Department for Business, Energy and Industrial Strategy said its assurances to Nissan had created no new contingent liabilities.

The autumn statement included £390m of investment in transport technology including driverless cars and low-emission vehicles, in which Nissan has taken a lead.

David Bailey, professor of industry at Aston university, said: “A lot of support for electric cars could be a way of favouring Nissan. Whatever Nissan got, and we don’t know what they got, other car manufacturers will be after it as well.”