The UK told Nissan it would seek tariff-free access to EU markets for the motor industry as part of Brexit talks, the business secretary has said.
Greg Clark said this statement of the government's objectives had helped persuade Nissan to boost its investment in the UK, securing thousands of jobs.
Ministers have been under pressure to clarify the "support and assurances" Nissan said it received.
Labour said Nissan had been told more about the Brexit strategy than MPs had.
Opposition parties have called on the government to spell out to Parliament what it wants to achieve from its Brexit talks before they formally begin.
And speaking on the BBC's Andrew Marr show, Mr Clark was asked how he had persuaded Nissan to build the new Qashqai and the X-Trail SUV at its Sunderland base.
The Japanese company's commitment to Britain's biggest car plant had been in doubt following the referendum on EU membership.
Mr Clark said he had assured Nissan that Britain would be "a great place to do business in the future".
Analysis: By Tom Bateman, BBC political correspondent
The business secretary was unexpectedly candid after days of calls for clarity about what guarantees the government had given the car maker.
As Mr Clark detailed the letter he wrote to Nissan's chief executive he provided further insight into the government's approach to its Brexit negotiations - at least insofar as the car making industry was concerned.
He signalled that ministers would seek tariff-free trade with other EU countries, and would provide support to ensure the industry remains "competitive" in Britain.
All this sounded a lot more like the "running commentary" on Brexit the government had been trying to avoid - a feature that may become more common as ministers come under pressure to ward off business uncertainty ahead of the negotiations.
While formal negotiations with the EU have not yet begun, he said: "I was able to convey what our demeanour would be, in those negotiations."
He said it would not be in either side's interests for tariffs to exist in the motor industry, adding: "So what I said is that our objective would be to ensure that we have continued access to the markets in Europe and vice versa without tariffs and without bureaucratic impediments and that is how we will approach those negotiations."
Pressed on the contents of the key letter he sent to Nissan, Mr Clark said that as well as seeking a "common ground" in Brexit talks, it included commitments to continue to make funds available for skills and training, to "bring home" elements of the supply chain which had migrated overseas, to support research and development, and to keep the UK car industry competitive.
He suggested the assurances would apply to the whole of the UK's car sector but added that "it's not general".
Since the announcement of Nissan's investment on Thursday, Labour has been calling for details of the promises made to the company and for similar assurances to apply to other industries.
Labour's shadow Brexit secretary Sir Keir Starmer said it was "extraordinary" that Mr Clark had revealed more about the government's Brexit plan than it had disclosed to Parliament.
He told ITV's Peston on Sunday: "It's not just this deal that we need to know about - it's what happens to the other businesses.
"Businesses are talking to me all of the time and they are very worried about what happens to them.
"They want to trade on the same terms and if there is a deal that's good enough for Nissan they are saying, and it's quite understandable, 'well, we want broadly the same deal for us'."
Mr Starmer said the government was refusing to reveal its Brexit plan to MPs, "but they told Nissan part of the plan and it's in a letter and the sooner we see that letter the better".