British Prime Minister Theresa May has written to the DUP saying the European Union wants to put a customs border in the Irish Sea in the event of a no-deal Brexit, according to a report by The Times of London.
Mrs May has reportedly written to the Northern Irish party saying that the EU is still pushing for a so-called "backstop to the backstop" - such a move would avoid a hard border on the island of Ireland.
In the leaked letter seen by the newspaper, Mrs May said she could not accept any "circumstances or conditions" that would break up the UK into two customs territories.
DUP leader Arlene Foster said the letter "raised alarm bells for those who value the integrity of our precious union and for those who want a proper Brexit for the whole of the UK".
"It appears the Prime Minister is wedded to the idea of a border down the Irish Sea with Northern Ireland in the EU single market regulatory regime."
The DUP, which supports Mrs May's minority government at Westminster, has repeatedly said it will not support a border in the Irish Sea.
A Downing Street spokesman said the Mrs May's letter sets out her commitment to never accepting any circumstances in which the UK is divided into two customs territories.
"The government will not agree to anything that brings about a hard border on the island of Ireland," the spokesman said.
"There are already arrangements in place such as the single electricity market, and on animal health, that reflect the unique relationship between Ireland and Northern Ireland.
"We want to make sure nothing damages the integrity of the UK's own internal market, and that businesses in Northern Ireland can continue to access it as they do now, and that's what our proposals deliver."
Meanwhile, a meeting of the British Irish Council in the Isle of Man today will be attended by the Taoiseach Leo Varadkar, Mrs May's effective deputy prime minister David Lidington and Northern Ireland Secretary Karen Bradley.
The ongoing political deadlock in Northern Ireland will be a key area off discussion as will Brexit, which is now at a critical juncture.
Established under the Good Friday Agreement, the British Irish Council provides a forum for collaboration on issues of importance to the Irish and British governments and the devolved administrations.
Today’s meeting in the Isle of Man will also be attended by Scotland’s First Minister Nicola Sturgeon and the Welsh First Minister Carwyn Jones.
Since the collapse of the Stormont Assembly, Northern Ireland does not have political representation at the discussions.
Downing Street has played down suggestions that a Brexit deal is imminent, after European Council president Donald Tusk appeared to indicate a breakthrough could come within the next week.
A senior UK Government source said that reports in the European media that a deal could come in the next few days should be taken "with a very large pinch of salt".
A potential sticking point could be demands for EU fishing fleets to be given continued access to British coastal waters as the price for agreeing to Mrs May's UK-wide backstop, the Daily Telegraph reported.
A UK-wide customs deal would maintain quota-free and tariff-free access to European markets for the British fishing industry and in return the EU wants to keep continued access to UK waters for its trawlers, the newspaper said.
Additional reporting by Fiona Mitchell