Battle lines drawn as UUP becomes official opposition in Northern Ireland
The Assembly is to have its first official opposition since the Good Friday Agreement after the Ulster Unionists announced they would not take a seat in the power-sharing government.
Party leader Mike Nesbitt said the move heralded a new era for devolved politics in the region.
The 1998 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 DUP leader Arlene Foster and Sinn Fein's Martin McGuinness were re-appointed First Minister and deputy First Minister respectively.
"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 Fein 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 Alliance Party is expected to remain in the Executive.
SDLP leader Colum Eastwood said his party would wait until negotiations around a new programme for government were completed.
The UUP walked out of the power-sharing administration last autumn, when official opposition was not an option, amid a crisis sparked by a murder linked to the Provisional IRA.
Mr Nesbitt had said his party's return to the Executive table 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.
He 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 Fein an attractive option.
Mr Nesbitt 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 was one of the key architects of the Good Friday Agreement while the DUP opposed the deal.
Mr McGuinness accused Mr Nesbitt of a lack of leadership, claiming he had "repudiated" the UUP's Good Friday Agreement legacy.