Prince Philip hoped the reaction to the murder of his uncle Lord Mountbatten would lead the IRA to turn away from violence, a letter set to be auctioned has revealed.
Also set to be auctioned is a letter from former Prime Minister Margaret Thatcher in the wake of the Brighton Bombing. She told a journalist that "evil must not prevail" after the bombing which killed five people and seriously injured 31 others.
In his letter the Duke of Edinburgh wrote that he hoped the reaction to the death of Lord Mountbatten would lead the IRA to renounce violence.
"Let us hope that the great wave of revulsion against this senseless act of terrorism may yet help to bring a change of heart in those who believe that violence and brutality are the only solutions to their problems," he wrote.
Lord Mountbatten was holidaying at his Castle in Sligo on August 27 1979 when he was murdered by the IRA.
A bomb exploded on a Shadow V boat he was fishing on, while he was pulled alive from the water by nearby fishermen, he died from his injuries before being brought to shore.
Also killed was his 14-year-old grandson Nicholas Knatchbull and Fermanagh schoolboy Paul Maxwell, who was working as a boat boy while on holiday with his parents.
The letter came to light as part of the collection of the late actor Lionel Jeffries who wrote to a number of senior royals following the murder of Lord Mountbatten to express his sympathy.
He received replies on behalf of the Queen, Queen Mother and Princess Margaret, which are also being auctioned.
Jeffries was best known for performing in films including Chitty Chitty Bang Bang, The Railway Children and Murder Ahoy!
He passed away in 2010 and the letter is being auctioned by a relative.
The murder of Lord Mountbatten was a watershed moment for the royal family, he had mentored Prince Philip in his youth and was Prince Charles' favourite uncle.
It is believed Prince William named his son Louis after Lord Louis Mountbatten. Louis is also Prince George's third name.
Also going under the hammer is the letter from Mrs Thatcher to journalist Montague 'Monty' Modlyn.
Mr Modlyn wrote to the Prime Minister to express his sympathy after the Brighton Bombing on October 12 1984.
Among those killed were Conservative MP Sir Anthony Berry, while cabinet member Norman Tebbit's wife Margaret was left permanently disabled.
Mrs Thatcher sent Mr Modlyn a hand-written reply on 10 Downing Street letterhead paper.
"It was very good of you both to be at Brighton for the closing speech. The enormity of what happened is just coming home to us. But evil must not prevail," the Prime Minister wrote.
The letter comes from a private collector who obtained it after Mr Modlyn's death in 1994.
All of the letters will be auctioned by Dominic Winter, of Cirencester, Gloucestershire on May 28 and are expected to fetch a combined £650.