Correspondence between the Business Secretary and Nissan’s chairman will not be made publicly available – despite months of deliberations – until the car firm’s “commercial confidentialities” cease to exist, it has emerged.

Last year the Department for Business, Energy and Industrial Strategy (BEIS) came under intense pressure from cross-party politicians to publish the letter Greg Clark had sent to Nissan’s chairman Carlos Ghosn. Assurances provided in the letter are believed to have given the Japanese car giant the confidence to agree to build the new Qashqai and X-Trail models in Sunderland following Brexit.

But in response to information requested by The Independent through freedom of information laws, BEIS said they had made the decision not to make public letters between Mr Clark and Mr Ghosn, adding: “We consider the public interest lies in favour of withholding the information at this stage.”

In December, the department was deliberating whether or not there was sufficient public interest in releasing the document. The department expected to reach a conclusion by the end of 2016 but delayed the decision again by a further two months.

Theresa May hailed the decision by Nissan to stay after Britain’s exit from the EU as “fantastic news”, adding: “This vote of confidence shows Britain is open for business and that we remain an outward-looking, world-leading nation.”

It comes after Colin Lawther, a senior executive at Nissan, appeared before a Commons select committee this week and told MPs the firm would be constantly reviewing its decision to stay in Sunderland. “As those circumstances change, and we wouldn’t wait until the end of the process, we will continually review the decisions that we take, based on anything that materially changes,” he said.

Andrew Tyrie, a Tory MP who chairs the Treasury select committee, said: “I haven’t heard a good reason why this letter hasn’t been published.”

Rebecca Long-Bailey, the shadow Business Secretary, told The Independent that the “secrecy which has shrouded the deal with Nissan has left every business in the UK wondering why it does get the Nissan treatment.” She added: “Backroom and secret deals do not constitute an industrial strategy.

“All businesses need stability and transparency from Government in the way they will be supported during the Brexit process. The strategy we have seen from Government so far appears to be crisis led and haphazard, indeed it is the very antithesis of an industrial strategy.”

The Liberal Democrat leader Tim Farron added: “The Government are making secret deals behind closed doors with individual businesses. How can they give assurances to businesses based in Japan but not tell the British public their plans. Then, they find any way to skirt around transparency laws to make sure no document ever sees the light of day. It’s a scandal.

“The Liberal Democrats are demanding that the letter and any information, even in a redacted form, is publicly released. What have the Government got to hide? This whole sorry saga is an utter farce, this Conservative Brexit Government are a car crash.”

BEIS added the information requested “contains commercially sensitive information and we consider that the disclosure of that information would be likely to prejudice the commercial interests of Nissan.”

It continued: “We also recognise that there has been particular interest in Nissan’s decision in relation to its Sunderland plant and understanding the context of the decision. However, against the public interest, there is a stronger public interest in withholding the information to ensure the commercial interests of external companies are not damaged or undermined by disclosure of information which is not common knowledge and which could adversely impact current or future business.”