The Government will not publish the letter it sent to Nissan which led to the car manufacturer's decision to pour new investment into its Sunderland plant, Greg Clark has signalled.
Ministers have been under pressure to reveal the terms of its agreement with the Japanese car giant following last week's announcement that the company will build two new models at its plant in the north east, securing the future of 7,000 jobs.
The Business Secretary set out details of the Government's four-point approach to the automotive sector, including Brexit reassurances, in a letter to Nissan chief executive Carlos Ghosn.
But Mr Clark suggested to MPs he will not publish the letter due to the need to protect Nissan's commercially sensitive investment plans.
The Government has strongly denied suggestions it offered the firm a "sweetheart deal".
Mr Clark told the Commons the letter contained commitments to continue to make funds available for skills and training, to encourage smaller firms in the supply chain to locate in the UK and to support research and development.
It also contained a pledge to keep the UK car industry competitive post-Brexit.
Labour's shadow business secretary Clive Lewis demanded that the letter be published.
But Mr Clark said: "You asked me whether I would publish the correspondence.
"I have set out the information that I gave them and I would just say this to you.
"My responsibility, on behalf of the Government, is to encourage and to attract investment in this country and it's important that when companies of all types and in all sectors share with me their investment plans that are of information to their prospective competitors that they can be assured that they are not going to be disclosed to their competitors to their disadvantage.
"My objective is to obtain those investments but I'm very happy to answer today and to the select committee where I intend to appear at the invitation of the committee all of the aspects of it."
Mr Lewis welcomed the investment announcement as "fantastic news" but insisted ministers needed to give more details about how the deal was done.
He said: "Are we really to believe that Nissan are risking millions of pounds of investment and the success of its newest models on the basis of the Government's good intentions alone?
"If that is the case, then why have they been keeping their good intentions to themselves?
"The overwhelming impression until now has been that the Government has no strategy for Brexit.
"If we are now expected to believe that the Government not only has a strategy, but that it has a strategy so convincing that it's persuaded Nissan to stay without the need for any special guarantees, then why won't they tell us what they are?"
Mr Lewis joked that Mr Clark must have discovered "the Brexit equivalent of the philosopher's stone".
"Tariff-free market access with no concessions, readily agreed by all 27 EU countries, including Wallonia," he said.
"Surely that is a feat worth sharing?"
The shadow business secretary said it appeared the Government was willing to give assurances to particular companies while leaving the majority "in the dark".
"If you didn't offer Nissan a sweetener, then what have you got to hide?
"Show us the letter," he said.
Mr Clark ridiculed Mr Lewis for adopting a "spluttering old banger of an approach" to the issue as he joked his opposite number should "upgrade to a new model".
"I recommend a Qashqai," he joked.
The Business Secretary also set out his vision for securing continued tariff-free access to EU markets for British manufacturers.
He said: "In our negotiations to leave the EU we will emphasise the very strong common ground that there is, especially in the automotive sector, between ourselves and other EU member states in ensuring the trade between us can be free and unencumbered by impediments.
"A good deal for the UK can also be a good deal for other member states and that is how we will approach the negotiations."
SNP frontbencher Callum McCaig told Mr Clark: "I think you may have said a little bit more to the BBC yesterday than you have to the House today.
"I hope you're not joining the ever-growing list of secretaries of state who have been slapped down by the Prime Minister for expressing their personal opinions in terms of this."
He went on: "You said to the BBC yesterday that our objective would be to have continued access without tariff or bureaucratic impediments.
"Now, that's not been said today but I think that's correct.
"But if that objective is not realised, what will the cost be to the taxpayer in terms of a deal with Nissan?
"What will be the cost to make good those tariffs should they be imposed?"
Mr Clark, in his reply, noted: "I certainly haven't been slapped down, up, sideways or anything by the Prime Minister, I'm pleased to say."
He added components in the automotive industry going backwards and forwards between the EU and UK means there is a "clear common interest in having arrangements that are free of tariffs and free of the bureaucratic impediments that I mentioned".
Labour's Hilary Benn, chairman of the Brexit Select Committee, asked Mr Clark: "Can you tell the House whether your offer of tariff-free access to the European market will be available to all other parts of our manufacturing sector, given what you said a moment ago - and if I heard you right - you indicated the Government might take a different approach to different sectors?
"If that means some of them might not benefit from tariff-free access, they'd like to know pretty quickly."
Mr Clark, in his reply, said: "It's not in my gift to offer tariff-free access to the single market, it was what I was describing what would be a positive outcome from the negotiations and therefore the demeanour we should take in those negotiations."
He added his team will talk to companies across the country to "understand what is important to them" to inform the negotiations.