Legal advice forced Plaid Cymrus ruling body to backtrack on a recommendation that an MP who assaulted his wife should not be allowed to represent the party in the Commons.
Jonathan Edwards was cautioned after the assault at his home in 2020.
Senior figures wanted Mr Edwards to be kept out of the Plaid MPs group.
But the party announced on Wednesday that he would have the whip restored, despite confirming that a majority of its National Executive Committee (NEC) had recommend excluding him.
The same statement, issued by the chair of the party, Beca Brown, said Mr Edwards was reinstated as a Plaid MP.
BBC Wales has been told the party received legal advice that the NECs position was "unlawful".
Former leader Leanne Wood posted on Twitter that she was "disappointed" with the decision - she had called for Mr Edwards to be kept out of the party altogether.
Senior Plaid Senedd member Sian Gwenllian, who is a member of the NEC, said in a Nation.Cymru article that it had "become apparent that our partys disciplinary procedures do not sufficiently differentiate between ordinary members and members who have been elected to positions of influence".
She added: "So Jonathan Edwards can automatically resume his title of Plaid MP following the panels announcement".
Ms Gwenllian, who was among those who backed the NEC recommendation, wrote that her view had not changed, and that if an elected politician had "severely damaged the integrity of our party... then that person should no longer represent the collective, democratic voice of our members".
The party is now reviewing whether any lessons can be learned from what happened.
Details of the incident at the MPs family home in May 2020 are not known.
Mr Edwards, who is now separated from his wife, has expressed "deep remorse" for his behaviour.
"The past two years have been a period of deep reflection in which I enrolled on a domestic violence awareness course which has helped me understand the impact my action had on others."
Its clear that Jonathan Edwards return to Plaid Cymru has caused ructions within the party.
He has support from members of his constituency party and some high profile figures who say hes a good MP and that his expressions of remorse should be accepted.
But other senior people in the party say zero tolerance is the only way to eradicate the societally entrenched problem of domestic abuse.
Whichever way you look at it, the fact remains that a majority of the partys ruling executive did not want him back and had their hand forced by legal advice.
Both sides are now battening down the hatches and hoping the storm will pass, but this has the potential to be a damaging split within the party.
Hazel Evans, a Plaid councillor in Carmarthenshire, said Mr Edwards was a "brilliant MP" and welcomed the decision to return the whip.
"We have to have forgiveness somewhere," she said.
She added "something had gone wrong" when the partys national executive intervened in the disciplinary process.
Betsan Jones, a county councillor and chair of the party in Mr Edwards constituency, said there had been a lot of support for him locally.
"I support him and am pleased that hes had the whip restored to him and I think we can draw a line under this," she said.