Theresa May’s government has been forced to reveal a key part of its Brexit negotiation strategy after days of pressure over secret reassurances to Japanese car giant Nissan.

Business Secretary Greg Clark also admitted that he had given Nissan written details of the plan, to seek tariff-free access to European Union markets, in a letter.

The news prompted fury among opposition parties.

Read more: Sir Tom Devine - Brexit will never happen

Former Labour leader Ed Miliband accused the Tory government of planning to keep MPs in the dark while spilling the beans to business.

The position was “totally unsustainable”, he said.

Mr Clark's candour comes just days after high-level talks at Downing Street ended with First Minister Nicola Sturgeon expressing frustration that she was no closer to knowing the May’s government’s strategy for Brexit talks.

The row erupted last week after Nissan announced plans to invest in the production of two new models at its Sunderland plant, securing the future of 7,000 jobs in the UK's biggest car factory.

Chief executive Carlos Ghosn said that the decision had been made after his firm received "support and assurances" from the Government.

Ministers denied that the firm was offered a "sweetheart deal".

But pressure intensified over the weekend as MPs and the Scottish Government demanded to know what Nissan had been promised – and whether or not the same terms would be extended to other businesses.

Read more: Sir Tom Devine - Brexit will never happen

Scottish Government’s Brexit minister Michael Russell also suggested that if Nissan could have a special deal on the EU then so could Scotland.

Mr Clark said that his commitments, in a letter to the Nissan chief executive, also included continued funds for skills and training, support for research and development and a plan to encourage smaller firms in the supply chain to locate in the UK.

On the BBC's Andrew Marr programme he also confirmed that World Trade Organisation rules on state aid mean the Government could not offer to compensate Nissan if it was hit by new post-Brexit tariffs.

"It is simply not possible to compensate for any future risks so the intention of keeping the sector competitive was important," he said.

"What I said was that our objective would be to ensure that we would 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.”

He said that such a deal should suit both sides in the talks.

“Continental European car manufacturers.. export a lot to us, we export a lot to them... If you conduct the negotiations in a serious, constructive and civilised way there is a lot in common that we can establish.”

His comments will lead to speculation that the government wants to remain part of the EU customs union, though he refused to be drawn on that issue.

Already, however, there is a move against the outcome from a leading pro-Brexit group.

Read more: Sir Tom Devine - Brexit will never happen

Change Britain, co-founded by Michael Gove and endorsed by Boris Johnson, said that the UK could secure trade deals worth twice the amount of those signed by the EU if it adopted a more extreme form of Brexit and rejected the idea of a customs union

Last night Labour pushed the Tory Government to say whether or not there was a "financial element" to the agreement with Nissan.

Meanwhile, a new report suggests that Brexit might not pose a significant risk to the Scottish financial services sector.

The University of Strathclyde report by former Royal Bank of Scotland economist Jeremy Peat, and Owen Kelly, lecturer at Edinburgh Napier University Business School and former head of Scottish Financial Enterprise, found that even in the worse-case scenario of 'hard Brexit, these risks do not make a case for Scottish independence.

A Scottish government spokesman said that Brexit was "far and away the biggest threat to Scotland's economy, jobs and long-term prosperity".