Boris Johnson and his ministers have made huge promises for the United Kingdom after we leave the European Union next week, particularly for “the north” – that unfamiliar land which can now be mined for Tory votes, if no longer for coal.

Conservative MPs elected in former Labour strongholds have pledged to “hunt like a pack” for infrastructure improvements – of which there has been no sign over the past decade of Tory government. The north is promised bounty from the “shared prosperity fund” – the cynically named, back-of-an-envelope replacement for EU structural funds, which for decades have invested billions in Britain’s regions to encourage economic revival.

Transformation is promised for when Brexit is “done”, yet there is still no acknowledgement that the most severe economic threat to the north is Brexit itself.

The risks highlighted by government impact assessments – which Theresa May was so reluctant to publish – remain, with the north and Midlands facing the greatest peril due to the significance of their trade with Europe. And the chancellor Sajid Javid’s insistence that Britain will diverge from EU industrial and social standards puts the risk to the regional economy at the more serious levels calculated by those assessments.

Blighted as many areas have been by job losses and severe public spending cuts, it is nevertheless a caricature to depict the north and Midlands as barely rebuilt since the mines, shipbuilding and other heavy industries were dismantled under the Thatcher government in the 1980s. Efforts have been made over three decades to regenerate deindustrialised areas – including investment, consistently, from the now-disappearing EU funds that are specifically targeted at deprived areas of Europe. British policy has been patchy, never enough, and was further undermined from 2010 by Tory governments withdrawing swathes of funding, and abolishing regional development agencies.

Transformation is promised for when Brexit is 'done', yet there is still no acknowledgement that the most severe economic threat to the north is Brexit itself

Yet the modern industries that have been established during this period, of which car manufacturing is the strongest example, have based their operations on Britain being a member of the EU.

The north-east, a small region by population and still pockmarked by decline, has hosted the growth of Nissan, and the 30,000 jobs in its supply chain, since the Japanese car giant was first attracted to Sunderland in 1986. The impact assessments found that this region, with its high-quality manufacturers principally exporting to Europe, faces losing the most: an 11% hit even with a trade deal, if Johnson achieves the incredible and strikes one by the end of this year. If there is no deal, and full barriers and tariffs are erected with Europe, the projected hit is 16%.

The north-west – where Leigh, Burnley and Blackpool South voted for Johnson but where there are still strong chemicals, machinery and transport export industries – faces an 8% economic decline, rising to 12% if there is no trade deal. The damage to the West Midlands, east Midlands, and Yorkshire and Humber is projected to be at least 5%, and up to 13% for the West Midlands if there’s no deal.

These downturns would affect companies in all sectors, putting hundreds of thousands of jobs at risk – such as in the ceramics industry, which still directly employs 22,500 people, many in its traditional Staffordshire heartland. The new year has already delivered a bracing blow to 283 steelworkers in South Yorkshire, where Liberty Speciality Steels cited “issues around Brexit” among its difficulties when announcing the redundancies.

‘Johnson’s new MPs have gushed about knocking on doors in Sedgefield, Blyth Valley and Redcar, and finding people impatient for the Brexit they voted for in 2016.’ Boris Johnson visits Sedgefield after his election victory. Photograph: Lindsey Parnaby/AP

Johnson’s new MPs have gushed about knocking on doors in Sedgefield, Blyth Valley and Redcar, and finding people impatient for the Brexit they voted for in 2016. But when the MPs talk to people involved professionally in their constituencies’ economic development, they will find them almost unanimously alarmed by the prospect of leaving the EU.

In the north-east, the region’s CBI, Chamber of Commerce, Federation of Small Businesses, Entrepreneurs’ Forum, Northern TUC, local authorities and its four universities signed up to a call for continued access to the single market – which seems impossible now – in order to lessen the economic damage.

Nissan itself, which employs almost 8,000 people directly in Sunderland, gradually escalated the seriousness of its warnings, before saying emphatically last October that a no-deal Brexit would make its UK operation unviable. The day after Johnson’s election victory, a company spokeswoman said it is “still waiting for clarity on what the future trading relationship between the UK and the EU will look like”.

The Japanese government has repeatedly issued similar warnings on behalf of 1,000 Japanese companies based in the UK, saying some are already halting investment and looking to move operations to Europe. A survey cited by the Nikkei Asian Review found that more than 70% of Japanese manufacturers in Britain are already “feeling a negative impact from Brexit”.

The efforts of UK business organisations, trades unions and other experts to explain the EU’s benefits and the risks of leaving have been bulldozed by three-word slogans, preceded by years of misinformation, all gleefully fuelled by the current prime minister. He may never be held to account for that, or for his new pledges to the north, but as he now moves to get Brexit done, his promises are finally set to collide with reality.

• David Conn is a Guardian writer