DUP MP Emma Little Pengelli has been co-opted to replace her leader Sir Jeffrey Donaldson on Stormont benches.
Sir Jeffrey won a seat in the Lagan Valley, a constituency he represents in Westminster, just days after running for the Assembly.
However, earlier this week he indicated that he intends to remain in Westminster as an MP until problems with Northern Ireland The protocol is resolved.
The Northern Ireland Electoral Service confirmed on Thursday that Ms. Little Pengelly had been returned as an MLA for the Lagan Valley.
Sir Jeffrey has stated that his party will not nominate ministers to Stormont so far UK Government takes action on the protocol.
In his statement, he said: “The people of the Lagan Valley and the whole of Northern Ireland have given me a mandate based on a commitment that we will not nominate ministers for a new Executive of Northern Ireland until the problems arising from the Northern Ireland Protocol are satisfactorily resolved and Stormont is able to return on a stable and lasting footing.
“I will continue as Member of Parliament for the Lagan Valley to address protocol issues.
“It is important that the protocol be replaced by new arrangements that respect the place of Northern Ireland in the United Kingdom and as the leader of the union operating in London.
“In the interim, I asked Emma Little Pengelly to represent the people of Lagan County County in the Northern Ireland Assembly.
“Emma will be a first-class defender of the people and will work hard to promote issues that are important to everyone in the Lagan Valley.”
Ms. Little Pengeley said her party leader had “an important task to accomplish in resolving the long-running protocol problem.”
“It requires him to be in London and work closely with the UK government,” she said.
“So I agreed to take his place in the Lagan Valley at this time to support this work and provide a consistently high level of support and service to Lagan Valley voters.
“It is a privilege to be asked to fulfill this role at this important time for Uniatism and for Northern Ireland.
“While Sir Jeffrey is focusing on this urgent task, I look forward to serving the people of the Lagan Valley with passion and devotion.”
Unionists strongly oppose post-Brexit trade agreements as a border in the Irish Sea.
The Assembly of Northern Ireland is due to sit on Friday, for the first time since Sinn Féin became the first nationalist or Republican party to lead the election.
The first step will be to nominate a new speaker, without whom the Assembly cannot sit.
Earlier this week, Sir Jeffrey did not rule out that his party would refuse to nominate a speaker.
On Thursday, he said the party would decide to run for office on Friday.
Ms. Little Pengeley previously served in the party as a senior special adviser and was elected MP from South Belfast in 2015-17 before giving way to Claire Hannah of the SDLP.
She has recently worked as a lawyer in Belfast.