Sir Jeffrey Donaldson has rejected claims of "running scared" from a Westminster by-election after not taking up a vacant Stormont seat in Lagan Valley.
The DUP leader said the idea that he or his party was "running away from the electorate is for the fairies".
The party has co-opted councillor Paul Rankin to replace Edwin Poots after the Agriculture Minister switched to South Belfast following the sudden death of Christopher Stalford.
Read more: Christopher Stalford to be replaced by Edwin Poots in South Belfast, DUP leader announces
Mr Rankin will be in the role for just two weeks before the dissolution of the Stormont Assembly at the end of March for fresh elections.
Sir Jeffrey, who represents Lagan Valley at Westminster, confirmed he still intends to run as a candidate in the constituency for the Assembly election in May.
A move to Stormont would mean Sir Jeffrey would have to give up his MP seat, triggering a by-election.
The DUP leader rejected suggestions that he did not take up the vacant MLA seat due to fears his party could lose a future by-election in the constituency.
"I'm going to the electorate on May 5, I'm putting my name forward," he told the BBC.
"So the idea that I or the DUP are running away from the electorate is for the fairies.
"We're putting ourselves forward, seeking a fresh mandate from the people of Northern Ireland.
"We're putting our plan to the people of Northern Ireland and we're looking for their endorsement so that we can tackle the issues that matter to them."
In the 2019 Westminster poll, Sir Jeffrey's vote share plunged by 16% while second-place Alliance saw its vote surge by almost 18% with candidate Sorcha Eastwood.
Despite the shift, Sir Jeffrey still comfortably retained the seat with 19,586 votes (43.1%) compared to Ms Eastwood's 13,087 (28.8%).
Former Alliance leader David Ford accused Sir Jeffrey, who has been Lagan Valley MP for almost 25 years, of showing "zero" commitment to Stormont.
Mr Ford tweeted: "Looks like Jeffrey Donaldson has shown his level of commitment to the Assembly.
"That's level zero. Running scared from @SorchaEastwood and @allianceparty."
His message was shared on the social media platform by the current Alliance leader, Naomi Long.
Sir Jeffrey has previously said his party will not re-enter the Stormont Executive until its demands are met to address issues with Brexit's Northern Ireland Protocol.
Reacting to the Lagan Valley MLA announcement, loyalist activist Jamie Bryson tweeted: "Perhaps a sign the DUP know there's no way back for the Executive for a long time, and thus it makes more sense for Sir Jeffrey to remain at Westminster."
TUV Lagan Valley Assembly candidate Lorna Smyth said many will be asking, "Where's Jeffrey?"
She added: "Now that Sir Jeffrey has declined the opportunity to return as an MLA for the constituency he has represented at Stormont and Westminster for a quarter of a century people are entitled to ask why."
Sir Jeffrey would not be drawn on the number of candidates the DUP would field in Lagan Valley for the Assembly election, saying an announcement would be made at a later date.
Former First Minister Paul Givan and Mr Rankin are now the DUP's two MLAs for Lagan Valley.
Earlier in a statement Sir Jeffrey welcomed the co-option of Mr Rankin.
He said: "Paul has a sterling record of hard work and delivery for the people of this area, with over 20 years of experience as an elected representative.
"I have worked closely with Paul for many years to champion the case for Lagan Valley and its people. I know he will be a diligent and dedicated servant of their interests in the Northern Ireland Assembly."
Read more: Christopher Stalford to be replaced by Edwin Poots in South Belfast, DUP leader announces
Read more: Crossgar Protocol rally: Unionists urged 'wake up' & maximise vote at upcoming Assembly election
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