By Agence France-Presse
The correspondence between Kennedy and David Ormsby Gore, Britain's ambassador to the US during Kennedy's presidency, show her explaining her reasons for not wishing to wed the former diplomat.
"We have known so much & shared & lost so much together -- Even if it isn't the way you wish now -- I hope that bond of love and pain will never be cut," she wrote to Ormsby Gore after his proposal in 1968.
"You are like my beloved beloved brother -- and mentor -- and the only original spirit I know -- as you were to Jack," she added.
Ormsby Gore had suggested a "secret marriage" during the summer of that year and said he found the categorical rejection "unbearable".
His wife was killed in a car accident in 1967 and Kennedy, whose husband was assassinated while president in 1963, had written to offer her condolences.
"Your last letter was such a cri de coeur of loneliness -- I would do anything to take that anguish from you," she told the former ambassador.
Kennedy went on to marry Greek shipping mogul Aristotle Onassis, explaining her choice in her final letter to Ormsby Gore.
"You and I have shared so many lives and deaths and hopes and pain -- we will share them forever and be forever bound together by them," she wrote.
"If ever I can find some healing and some comfort -- it has to be with someone who is not a part of all my world of past and pain -- I can find that now -- if the world will let us."
The collection of 18 letters from Kennedy to Ormsby Gore covers the period from her husband's assassination until her marriage to Onassis in 1968.
They are part of a larger cache of papers found at Ormsby Gore's family home, including correspondence from John F. Kennedy and British prime ministers, which is expected to fetch up to 150,000 pounds ($190,000, 170,000 euros) at the sale by Bonhams auction house.
Ormsby Gore served as British ambassador from 1961 until 1965, when he returned to the UK, and died after a car crash in 1985.