A no-deal Brexit could result in electricity blackouts across Northern Ireland, government officials have warned.
Officials are preparing a 'no deal' notice about the impacts on the all-island Single Electricity Market (SEM).
They have suggested there are risks of price increases and supply shortages.
Most experts have considered the SEM to be a relatively unproblematic part of Brexit.
The reason is that it is a UK-Ireland bilateral arrangement, rather than an EU arrangement.
It has been in place since 2007 and allows for a single market to operate across Northern Ireland and the Republic of Ireland.
However, it is underpinned by the UK and Ireland's membership of the Internal Energy Market (IEM).
'Insecure, isolated NI market'
All EU members participate in the IEM along with Norway, Iceland and Lichtenstein.
Participation requires alignment with EU rules including industrial emissions regulations and restrictions on state aid.
The work carried out by UK officials says the surest way to maintain the SEM is for continued UK participation in the IEM.
They suggest that in a no-deal Brexit, the SEM would, at best, continue but be at risk.
They raise an alternative possibility that it could split "leaving an insecure, isolated NI market".
That in turn could see bills in Northern Ireland rise by up to 34%, lead to blackouts and prompt government intervention.
Keeping the SEM operating "in any way possible" is described as a priority.
The officials suggest that even if there is no withdrawal deal, the government could seek a special deal with Ireland and the EU to keep the SEM operating.
But they cautioned that such an arrangement would not fully mitigate legal risks to the SEM.
It is understood that a formal "no deal notice" on the SEM will be published in the next few weeks.
A spokesman for the Department for Exiting the EU said the government does not comment on leaks.
The blackout warning was met with scepticism by DUP MP Ian Paisley, a Brexit supporter, who dismissed it on Twitter as "fake news".
Mr Paisely tweeted: "Don't forget planes falling out of the sky, food shortage, mass unemployment and pestilence!"