The Scottish National Party has attacked the Government over an alleged deal with Nissan to keep its Sunderland plant safe.
Workers, unions, politicians and business leaders were overjoyed last week at news the Japanese car manufacturing giant would build its new Qashqai and X-Trail models at its plant in Washington, allaying fears Brexit would be a disaster for Nissan workers in the UK.
But opposition politicians have claimed the Government must have made a deal with Nissan to see the decision through.
Now the Scottish National Party has submitted a Freedom of Information request to obtain a letter sent by Business Secretary Greg Clark to Nissan on Brexit plans, and said it was "completely unsustainable" for ministers to resist publication.
SNP MSP Richard Lochhead said: "It is simply unacceptable that the UK Government is conducting backroom Brexit deals with multinational companies at the same time as they are keeping the public and devolved governments in the dark.
"People will naturally be relieved if jobs are being secured but other industries will now expect similar guarantees."
He added: "Sunderland voted overwhelmingly to leave Europe while Scotland voted decisively to remain - so if the Tories are only prepared to be flexible for places like Sunderland while ignoring Scotland's democratic voice, it would be an outrage."
Meanwhile, business leaders have been warned by a Cabinet member to "assume nothing" about whether the UK will remain in the EU customs union after Brexit.
Work and Pensions Secretary Damian Green issued the caution as the fall-out from the Nissan investment deal continued to put the Government's plans for the tough "divorce deal" negotiations with Brussels under intense scrutiny.
The comments came after Business Secretary Greg Clark insisted a key objective in the negotiations will be to secure continued tariff-free access to the EU market for British manufacturers.
With experts stating this would imply the UK remaining in the EU customs union, and therefore unable to cut international trade deals of its own, critics accused the Government of creating confusion for business.
Pressed on whether the UK would remain in the customs union, Mr Green told BBC Radio 4's Today programme: "Assume nothing at the moment because, I think, a lot of this discussion tends to be fairly simple, fairly black and white."
Mr Green said Nissan had decided to boost UK production because the Government convinced it the country will remain competitive after Brexit.
"We are able to tell Nissan our stance towards both the renegotiations and, perhaps, even more importantly, our stance towards making Britain a great place to do business for the years ahead, in a way that clearly satisfied them, that they want to continue to be the biggest car maker in this country.
"They know the negotiation we are going to have with the EU will leave Britain open for business, will leave us competitive," he said.
At the weekend, Mr Clark refused to be drawn on whether the Government was seeking to remain part of the EU customs union or on the implications for its commitment to end free movement of labour with the EU, saying it had yet to fully "crystallise" its negotiating position.
Theresa May's official spokeswoman told a regular Westminster media briefing that the Prime Minister had made clear in the House of Commons that she did not regard membership of the Customs Union as a "binary choice" and that she would be looking at getting "the best deal for the UK" rather than accepting the arrangements already in place for other countries.
Asked whether this might mean seeking special deals for different parts of the economy, such as the automotive industry, the spokeswoman said: "We are looking very carefully at different sectoral issues.
"That is why there is such a level of engagement across government with different sectors to prepare, with the principle being that we want to make sure businesses can continue to trade freely."
Asked whether the Government would make public Mr Clark's "letter of comfort" setting out assurances to Nissan, the spokeswoman said: "I'm not sure we get a lot from asking for every single letter that the Business Secretary does when he engages with business to be published."