Tonga's nobility has chosen Lord Nuku to replace the former Speaker, Lord Lasike, who was barred from parliament last month after being convicted on criminal charges.
Lord Nuku fills the vacant Eua seat.
He is a former cabinet minister, having served as Works Minister in the Lord Sevele government.
Meanwhile the opposition Democratic Party, which has been pressing for a vote of no confidence for more than six weeks, is expected to present its case from this afternoon.
One of their supporters, Uliti Uata, who left Cabinet to back the motion for a vote of no confidence, was admitted to hospital earlier this week.
The Tonga prime minister, Lord Tu'ivakano, has spent a week and a half putting his case that the motion has no merit.