British Prime Minister Boris Johnson will come face to face with criticism in Belfast next week after pro-UK unionists on Friday vetoed the start of a new, nationalist-led legislative assembly.

In a historic development, the role of Northern Ireland's first minister is set to be taken by the pro-Irish party Sinn Fein, after it triumphed in elections to the Stormont assembly last week.

But the pro-UK Democratic Unionist Party (DUP), angered at the "Northern Ireland Protocol" agreed as part of Britain's Brexit deal with the European Union, blocked the election of a speaker at Stormont.

The DUP's former first minister in the power-sharing executive between unionists and nationalists, Paul Givan, said his party was "determined to succeed" in removing the protocol.

Sinn Fein's Northern Ireland leader Michelle O'Neill accused the DUP of holding the British-ruled territory to "ransom".

"I've spoken with Boris Johnson himself. He will be here on Monday. I intend to put it to him directly that he needs to stop pandering to the DUP," she told reporters.

The UK government was "playing a game of chicken with the (European) commission right now, and we're caught in the middle", the first minister-elect added.

There was no immediate comment from Downing Street, after reports said Johnson's government was poised next week to suspend the EU protocol or legislate to eliminate its requirements from UK law.

The protocol mandates checks on goods coming to the province from mainland Great Britain, to ensure no return of a physical border between Northern Ireland and EU member Ireland to the south.

The elimination of the hard border was a key strand of the 1998 Good Friday Agreement, which ended three decades of sectarian violence in Northern Ireland.