The Irish Congress of Trade Unions, which represents 800,000 workers on both sides of the border, has given its backing to the draft deal agreed last week for the UK's departure from the European Union.
The leaders of the unions affiliated to the ICTU met in Belfast this morning and agreed to support the draft agreement.
Owen Reidy, ICTU Assistant General Secretary, said the draft deal was a poor one, but it was better than the UK crashing out of the EU without agreement.
"In the absence of any other alternative, this deal should be considered," he said.
Business and industry leaders in Northern Ireland, and the agriculture sector, have already given their backing to the draft deal, and are due to confirm their support when they meet Mrs May in London tomorrow.