SINN Féin leader Mary Lou McDonald has apologised for the 1979 assassination of Lord Mountbatten by the IRA.
The apology came the day after Prince Philip - who was Lord Mountbatten's nephew - was laid to rest in London following his death at the age of 99.
Ms McDonald described the killing as "heartbreaking" and said she was "sorry it happened."
"I am happy to reiterate that on the weekend that your Queen buried her beloved husband," she told Times Radio on Sunday.
It marks the very first time that a political leader of Irish republicans has apologised for the murder.
Gerry Adams, McDonald's predecessor, refused to apologise for the killing when he met Prince Charles, Philip's son, in 2015.
The IRA, of which Sinn Féin was once the political wing, killed Lord Mountbatten by blowing up his yacht while it was moored off the coast of Co. Sligo.
The attack was orchestrated by the IRA at the height of The Troubles, when conflict between Irish republicans and those who wanted Northern Ireland to remain in British hands was rife.
The IRA had repeatedly threatened to assassinate him, and nearly did a year before they succeeded when a sniper tried to fire at Lord Mountbatten from a boat, but was unable to take the shot due to bad weather.
The successful assassination attempt involved a 50 pound remote-control bomb which was hidden underneath the hull of Mountbatten's fishing boat.
When Mountbatten and his party had taken the boat just a few hundred yards from the shore, the bomb was detonated. The boat was destroyed by the force of the blast and Mountbatten's legs were almost blown off.
Mountbatten, then aged 79, was pulled alive from the water by nearby fishermen, but died from his injuries before being brought to shore.
Also killed in the blast were two teenage boys and an 83-year-old woman, all relatives of Mountbatten's who were out fishing with him.