The former Ulster Unionist MP, Jeffrey Donaldson, has confirmed his defection to the Democratic Unionist Party.
Mr Donaldson resigned from David Trimble's Ulster Unionists before Christmas, having put in a poll-topping performance in his Lagan Valley constituency at the Assembly election.
He had been involved in an ongoing battle within the party over its policies on the Good Friday Agreement.
Mr Donaldson makes the move to the Ian Paisley-led DUP along with another Lagan Valley MLA, Norah Beare, and the Fermanagh South Tyrone MLA Arlene Foster, who resigned from the UUP at the same time.
The announcement is another setback for David Trimble. The DUP overtook the UUP in the November election to become the main voice in unionism.
The DUP captured 30 seats but the defections increase its share of Stormont seats to 33. This puts them nine ahead of the UUP, which has 24 seats.
The balance favours the DUP over the UUP in the number of ministers each party would be entitled to in any future Executive.
Reverend Ian Paisley has described the defection of Mr Donaldson and his colleagues as a historic day for unionism. He welcomed the trio to the DUP Assembly team and predicted further defections.