Northern Irish kingmakers warn PM May: Don't sell out the United Kingdom

The Northern Irish party which props up Prime Minister Theresa May's government warned her on Friday that her negotiation had raised alarm bells and that it could not support a Brexit deal that divided up the United Kingdom.

Less than five months before Britain is due to leave the EU on March 29, a deal is 95 percent done but officials caution they are still haggling over the fate of the land border between British-ruled Northern Ireland and EU member Ireland.

The Times newspaper reported that the EU wants a customs border in the Irish sea in the event of a no-deal Brexit, citing a leaked letter from May to the Northern Irish party which props up her government.

"The Prime Minister's letter raises 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 United Kingdom," Democratic Unionist Party (DUP) leader Arlene Foster said.

"From her letter, 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," Foster said.

The DUP has caused friction before: nearly a year ago, a refusal by the DUP to sign off on a deal on the border caused the temporary collapse of Brexit talks at a crucial stage, before negotiators found a way to keep all sides on board.

The Brexit deal - or the lack of one - will shape Britain's prosperity for generations to come and have long-term consequences for the European Union's global clout.

Both sides need an agreement to keep trade flowing between the world's biggest trading bloc and the fifth largest global economy. The other 27 members of the EU combined have about five times the economic might of Britain.