A letter from Prince Philip revealing that he hoped the reaction to the murder of his uncle Lord Louis Mountbatten by the IRA would help bring peace to Northern Ireland has gone up for sale in an online auction. 

Another letter from Margaret Thatcher shortly after she was targeted by the IRA in the Brighton Bombing is also set to be auctioned later this month. 

Lord Mountbatten was assassinated by the IRA in 1979 when the terrorist organization blew up his fishing boat off the coast of Sligo. The attack also claimed the life of Mountbatten's teenage grandson - Nicholas Knatchbull -  and the life of a teenage crew member - Paul Maxwell -  from Enniskillen in County Fermanagh. 

Mountbatten was pulled from the waters alive by a local fisherman but died shortly afterward from his injuries. 

The Royal Family received dozens of letters of condolences following Mountbatten's murder and among the sympathizers was actor Lionel Jeffries, who Prince Philip is responding to in the letter to be auctioned.

Read more: FBI files allege Lord Mountbatten, murdered by the IRA, was a pedophile

Prince Philip said that he hoped the widespread outrage and condemnation that greeted Mountbatten's murder would cause the IRA to have a change of heart and abandon their campaign of violence in Northern Ireland. 

"I am most grateful to you for your thoughtful letter of sympathy," Prince Philip said in the letter. 

"Let us hope that the great wave of revulsion against this senseless act of terrorism may yet bring a change of heart in those that believe that violence and brutality are the only solutions to their problems." 

Jeffries died in 2010 and the letter passed into the possession of one of his relatives. The relative is auctioning that letter and four other secretarial letters written on behalf of other members of the royal family later this month. 

Margaret Thatcher, meanwhile, penned a letter to journalist Montague Modlyn shortly after the Brighton Bombing almost claimed her life. 

Modlyn wrote to Thatcher to express his sympathies following the bombing of the Conservative Party Conference in the Grand Brighton Hotel in October 1984. The blast killed five people, including a Conservative MP. 

Thatcher wrote a hand-written reply to Modlyn and said that evil would not prevail. 

"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," she said in the 1984 letter. 

Both letters and the secretarial responses are set to be auctioned by Dominic Winter Auctions on May 28. 

The set is expected to fetch around £600 ($725).

Read more: “Berserk” Margaret Thatcher admitted she was losing war against IRA in 1988