The Ulster Unionist Party has announced it will not be taking part in Northern Ireland's power-sharing administration and will go into formal opposition for the first time since the Good Friday Agreement.
The SDLP has yet to decide if it too will opt out of government with party leader Colum Eastwood saying his party will wait to see if a programme for government can be agreed before it decides.
UUP leader Mike Nesbitt said the party would take up the recently established option of entering formal opposition to the coalition executive.
"Let the battle commence," Mr Nesbitt told the Assembly chamber.
Meanwhile, DUP leader Arlene Foster has been elected first minister of the Northern Ireland Assembly following last week's election.
Ms Foster has held the position since January after replacing the retiring Peter Robinson and led the DUP as it was returned as the largest party in the Assembly elections.
Sinn Féin's Martin McGuinness was re-elected deputy first minister, a position he has held since 2007.
Ms Foster and Mr McGuinness now have two weeks to hammer out a programme for government for a new ministerial executive, which will aim to create 50,000 jobs while dealing with a legacy of the Troubles as well as the demands of bereaved relatives for truth and justice.
The allocation of ministerial portfolios did not take place today.
The Ulster Unionist Party's position on whether to return to the Executive after a hiatus following the killing of Kevin McGuigan by members of the Provisional IRA last summer was in doubt until today.
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 parties to work collaboratively across departments.
He 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.
An independent examination ordered by the government said all the main Troubles paramilitary organisations retained structures, though their leaders were committed to the peace process.
Belfast has witnessed three shootings over 24 hours this week, culminating in the fatal shooting of takeaway delivery driver and father-of-six Dan Murray.
A terror attack in Britain by dissident republicans is now a "strong possibility", according to a new security assessment.
In her acceptance speech, Arlene Foster said she wants to make Northern Ireland a better, safer and more prosperous place.
She said she will work with other parties and is incredibly proud to be first minister.
In his address to the chamber, Mr McGuinness said the challenge for the Assembly is to recognise the importance of working together and delivering for the people the chamber represents.
He said he wants members to stand up for the poor, the marginalised and for people who feel they are not being treated fairly.
Robin Newton appointed Speaker of the Assembly
The DUP's East Belfast MLA Robin Newton was appointed Speaker of the Assembly succeeding Sinn Féin's Mitchel McLaughlin, who did not contest the recent elections.
In last week's election, the DUP maintained the 38 seats it held in the last assembly, while Sinn Féin lost one and now holds 28.
The UUP has 16 seats, while the SDLP lost two and now has 12.
The Alliance Party secured eight seats during the election, meaning it does not have enough seats to automatically qualify for a ministerial department.
