British Prime Minister Boris Johnson's Northern Irish allies have thrown Brexit into sudden disarray - again - just as it appeared he was about to shake hands on a divorce deal with Brussels.

In a rerun of Theresa May's Brussels nightmare a year ago, the Democratic Unionist Party announced they cannot support a draft divorce deal that was hammered out by diplomats over sleepless nights this week.

Johnson had been expected to announce the deal had been done on Thursday morning in Brussels at the opening of a two-day summit of EU leaders. However it was unlikely he would sign the existing draft text without the DUP on board.

The development all but guarantees there will be another delay to Brexit, requiring Johnson to break his promise of a "do or die" Brexit by October 31 and send a letter by this Saturday requesting an extension.