The business secretary says he will not publish his letter to Nissan because it contains sensitive commercial details.
Greg Clark told MPs companies had to be confident their plans would not be shared with their competitors.
Nissan has said two new car ranges will be built in Sunderland, saving thousands of jobs, after “support and assurances” from the government about the UK’s future outside the EU.
Labour attacked the “backroom deals” and demanded the letter’s release.
The building of the Qashqai and the X-Trail SUV in Sunderland had been in doubt following the Brexit vote.
In a Commons statement, Mr Clark repeated that he had assured Nissan the UK would be seeking trade that was “free and unencumbered by impediments” for the motor industry after Brexit.
He said the carmaker’s announcement was a “massive win” for 7,500 direct workers and the wider supply chain, adding: “It is hard to think of more unambiguously good news.”
Labour’s shadow business secretary Clive Lewis welcomed the investment, but said MPs and the government were being kept in the dark on the deal – and the government’s post-Brexit plans – despite Mr Clark revealing “tantalising details” in a BBC interview the previous day.
“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 intentions alone?” Mr Lewis asked.
He demanded to know what the government’s plans were for other industries like steel, aerospace and pharmaceuticals.
He told the minister: “If you didn’t offer a sweetener what have you got to hide – show us the letter.”
Mr Clark has previously said there was “no cheque book” involved in the assurances given to Nissan.
Replying to Mr Lewis on the subject of his letter to the carmaker, he said: “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.”
He repeated what he said were the letter’s key points: continued support for the competitiveness of the car industry, bringing more of the supply chain into the UK, backing for research and development, and seeking “unencumbered” trade.
He also accused Labour of a “miserable” reaction to Nissan’s announcement, asking: “Is it beyond him to put party politics aside?”
Greg Clark stands by refusal to publish Nissan letter}