LONDON — Seeking to salve an old wound at a time of sorrow for Britain’s royal family, the political leader of the Irish republican movement apologized Sunday for the 1979 assassination of Louis Mountbatten, an uncle of Prince Philip.

Mary Lou McDonald, the leader of Sinn Fein, which was once the political wing of the underground Irish Republican Army, told a London radio station, “Of course, I am sorry that happened; of course, that is heartbreaking.”

McDonald offered the landmark apology a day after Queen Elizabeth II buried her husband, Philip, in a ceremony at Windsor Castle that paid tribute to his military career. His uncle Lord Mountbatten, a celebrated commander during World War II who later served as the last viceroy of India, overseeing its partition and transition to independence, was killed after a bomb exploded on his fishing boat off the coast of Ireland.

The assassination, carried out by members of the IRA, was one of the highest-profile attacks during the Northern Ireland Troubles, and the one that struck closest to the heart of the royal family. In addition to his ties to Philip, Lord Mountbatten was friendly with the queen and a mentor to the couple’s eldest son, Prince Charles.

Speaking to Times Radio, McDonald said Sinn Fein was now engaged in peaceful politics. Asked if she would apologize to Charles for the assassination, she said that British armed forces had carried out violent acts in Northern Ireland but added that she regretted the killing of Lord Mountbatten.

“It is all our jobs to ensure no other child, no other family, no matter who they are, suffers the same trauma and heartbreak that was all too common on all sides of this island and beyond,” McDonald said. “I am happy to reiterate that on the weekend that your queen buried her beloved husband.”

Lord Mountbatten was vacationing at his Irish home, Classiebawn Castle, in August 1979 when the IRA placed a radio-controlled bomb on his boat, detonating it when he was several hundred yards offshore, lobster potting with members of his family.

In addition to Lord Mountbatten, the explosion killed Lady Brabourne, the mother-in-law of his elder daughter, Patricia; his 14-year-old grandson, Nicholas Knatchbull; and a friend, Paul Maxwell.