The Northern Ireland Assembly is to have its first formal opposition since the Belfast Agreement after the Ulster Unionist Party announced it would not take a seat in the powersharing government.

Party leader Mike Nesbitt said the move heralded a new era for devolved politics in the region.

The 1998 Belfast Agreement established a form of government based on a ruling coalition Executive made up of all Northern Ireland’s main parties. The aim was to ensure all sections of a deeply divided society had a role in power.

While smaller parties and Independents have sat outside the Executive in past mandates, they have not been afforded the recognition, funding and status of an official opposition.

A new law passed earlier this year now enables parties with the electoral strength to enter the Executive to instead form an opposition.

Mr Nesbitt made the announcement moments after Democratic Unionist leader Arlene Foster and Sinn Féin veteran Martin McGuinness were reappointed First Minister and Deputy First Minister respectively in the first sitting of the legislature since last week's Assembly election.

“Let battle commence,” the UUP leader and former TV broadcaster told the Assembly chamber.

The UUP nailed its colours to the mast two days after receiving a proposed programme for government document and two weeks before the deadline for a new five-year government plan to be agreed and Executive formed.

With the DUP and Sinn Féin having consolidated their positions at the head of the Executive in the election, the focus now shifts to the SDLP to see if it will follow the UUP out of government. The cross-community Alliance Party is expected to remain in the Executive and again take on the politically sensitive justice portfolio.

SDLP leader Colum Eastwood said his party would wait until negotiations around a new programme for government were completed.

“We will make a decision when that is finished - only then,” he said.

“Mike Nesbitt will do what Mike Nesbitt will do. He has a mandate for the Ulster Unionist Party, I have a mandate for the SDLP’s position, which was always to negotiate hard to try to achieve a programme for government that we can all sign up to. That is our position. That remains our position.”

The UUP walked out of the powersharing administration last autumn, when official opposition was not an option, amid a crisis sparked by a murder linked to the Provisional IRA.

‘Progressive’ programme

Mr Nesbitt had said his party’s return to the Executive table post-election would depend on a number of factors, among them the need for a “progressive” programme for government and a commitment from the main parties to work collectively across departments.

He said neither had materialised.

Mr Nesbitt said an assessment by police chiefs that structures of the Provisional IRA remain in place also did not make a return to power with Sinn Féin an attractive option.

The UUP leader said his party’s Assembly group had voted unanimously to form an opposition.

"This heralds a new era for devolved politics at Stormont, and a big, bold step forward to normal democracy for Northern Ireland," he said.

Eighteen years ago, the UUP, the then dominant force within unionism, was one of the key architects of the Belfast Agreement, while the DUP opposed the deal.

Mr McGuinness accused Mr Nesbitt of a lack of leadership, claiming he had “repudiated” the UUP’s Belfast Agreement legacy.

“I find that deeply disappointing,” he said.

“I do think rather than being seen as leadership, it will be seen as a lack of leadership - it will be seen as a lack of the Ulster Unionist Party’s ability to accept the democratically expressed wishes of the people who have charged both the DUP and Sinn Féin with responsibility to lead this administration forward.”

DUP Assembly member Paul Givan said the UUP had been rejected by the electorate.

“With Mike under internal pressure he is now running into opposition, having lost the election,” he said.

He added: “Whilst others have run away, the DUP, as the leaders of unionism, will get on with the business of government.”

As an official opposition, the UUP will have additional speaking rights in the chamber, be able to table opposition day debates, will fill some key scrutiny committee roles and have access to funded research services.

Alliance Party leader David Ford accused Mr Nesbitt of "grandstanding".

“Whether in opposition or not, today’s move by the UUP shows Mr Nesbitt clearly makes decisions based on where he can best be seen, as opposed to what can best help the people of Northern Ireland,” he said.

Press Association