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A former commander of the Irish Republican Army who is now the deputy leader of Northern Ireland will join a historic state visit hosted by Britain's Queen Elizabeth II Tuesday.

Martin McGuinness joined the IRA in the 1970s and became a key figure in the paramilitary organization.

The queen's cousin, Lord Louis Mountbatten, was killed by an IRA bomb on his boat in 1979. He was one of more than 3,000 people killed in what was known as "The Troubles" between Catholics and Protestants over whether Northern Ireland should remain in Britain or become part of a unified Ireland.

Even when McGuinness was elected to the British parliament in 1997 under the IRA's political wing, Sinn Fein, he refused to take his seat in London because it would involve swearing an oath to the queen.

It is because of this fraught history that his attendance at a banquet hosted Tuesday by the queen is seen as such a significant event. The occasion is part of an equally symbolic four-day visit to the U.K. by Irish Prime Minister Michael D. Higgins, the first in history by an Irish head of state.

The queen became the first British monarch to visit Dublin in 2011, but Sinn Fein did not take part.

Ahead of the Tuesday trip Higgins said relations between the U.K. and Ireland were at a high but warned that securing peace in Northern Ireland would take "significant work," the BBC reported. He added that it would be wrong to try to "wipe the slate clean."

Sinn Fein leader Gerry Adams released a statement confirming McGuinness' attendance: "While Martin McGuinness's involvement in President Higgins's State visit may not be welcome by opponents of change, it is yet another example of Sinn Fein's commitment to an inclusive future based on tolerance and equality."

McGuinness was an IRA negotiator in deliberations to end :"The Troubles" and is credited with helping reach the Good Friday Agreement in 1998. This week's visit comes after his historic handshake with the queen during her visit to Northern Ireland in 2012, seen as an important milestone in the reconciliation efforts.

Britain's Queen Elizabeth II shakes hands with Northern Ireland Deputy First Minister Martin McGuinness in Belfast, Northern Ireland, on June 27, 2012.PAUL FAITH / AFP - Getty Images, file
-Alexander Smith