A letter from British Prime Minister Teresa Mae told the Times that the EU wants to impose a customs border in the Irish Sea between Northern Ireland and the rest of the UK if no agreement is reached for Britain's withdrawal from the bloc.
In the letter, May said she would not allow the division of Northern Ireland and Britain, while Arlen Foster, leader of the Unionist Democratic Party of Northern Ireland, said the message sounded alarm bells.
Mai is struggling to reach a negotiated withdrawal agreement in Brussels, within her deeply divided party and members of the Democratic Unionist Party, who do not want Northern Ireland to be treated differently from the rest of the UK.
A spokesman for the May office at Ten Downing Street said the letter specified May's commitment not to accept any situation in which UK territory was divided into two customs zones.
