The Government sent a letter promising to "pull out all the stops" for businesses such as Nissan in the event of a Brexit vote, ITV News has learned.
Both sides deny any kind of 'sweetheart deal' - but a senior government source told ITV News the letter promised to "support competitiveness" post-Brexit.
Political Correspondent Carl Dinnen said the letter is apparently not specific to Nissan - or even the car industry.
And what this guarantee of competitiveness actually means has been kept vague. But it is enough for Nissan to think it's worth staying.
The news, which will secure thousands of jobs, is the first major UK automotive decision since the Brexit vote in June.
Colin Lawther, Nissan's senior vice president for manufacturing in Europe, denied the company had received any written assurances from the Government.
He said: "There's no special deal for Nissan. We are working within the whole of the automotive industry.
"We would expect nothing for us that the rest of the industry wouldn't be able to have access to."
He added: "Clearly there is this massive basket of unknowns from triggering Article 50 to the end of the process, and it is a massive amount of potential outcomes, so it is not really productive for us to speculate on what could happen.
"We just trust that the Government will work through diligently to make sure, that as a whole industry, we end up competitive."
The Sunderland plant, which has been active since 1986, employs almost 7,000 people, producing around 2,000 cars a day.