The documents, seen by The Guardian, revealed that electricity bills in Northern Ireland could rise by up to 200 pounds ($262), and predicted the "collapse" of energy companies.
The Republic of Ireland and Northern Ireland have had a joint electricity market, known as the Single Electricity Market (SEM), for more than ten years. But the SEM is likely to collapse if the United Kingdom exits the European Union without a deal, the newspaper reported.
The issue of Northern Ireland remains a thorny issue in Brexit talks. May has repeatedly stressed that London would not tolerate any border separating Northern Ireland from the rest of the United Kingdom.