The secret letters of former U.S. President John F. Kennedy to his Swedish mistress, Gunilla von Post, have been put up for sale online.

One autograph signed letter and two partial handwritten letters, all dated between 1955 and 1956, are currently up for sale on  RR Auction.

After meeting in Cannes, France in 1953, JFK is said to have written to the aristocrat ahead of his marriage to Jacqueline Lee Bouvier.

Reports said Von Post disclosed their affair, included a week-long trip to Sweden in August 1955, in her book  Love, Jack published in 1997.  

"I was planning to come back again next summer to see you … and now what will happen. In any case let me know what you are going to do," the 1956 letter said.

"If you don't marry come over as I should like to see you," the letter added.

Kennedy, then a Massachusetts Senator, continued that he had a wonderful time last summer with Von Post. "It is a bright memory of my life – you are,e wonderful and I miss you," he added.

The auction house said this is the only JFK letter they had offered that shows concern to another woman when he was married.

In the partial 1995 letter, he wrote that Congress "will not finish until Aug.5." So Kennedy had to sail on the 'United States' and arrive on Aug. 10 at Le Havre. He added he would be in Sweden "on the 12th."

"Where do I go? Send me your address at Bastaad where you shall be," he said.

Many of the hidden letters had previously been put up for auction on a Chicago online auction site, with bidding starting at $25,000 for the artifacts.

The notes are all printed on official government paper, with each one bearing the letterhead of the United States Senate.

The auction house is also selling the second partial letter signed "Jack." The partial letter said "…coming and perhaps you could make me a reservation. I am anxious to see you."

 “Is it not strange after all these months? Perhaps at first it shall be a little difficult as we shall be strangers – but not strangers – and I am sure it will all work and I shall think that though it is a long way to Gunilla – it is worth it.”

The auction house said the collection of letters was found in Von Post's belongings after her death in 2011 and came from her property.

According to their website, RR Auction was created in 1976 and is a "globally recognized and trusted source for rare documents, books, autographs, and historic artifacts" based in Boston.

A personal lighter and cigar, a signed portrait of JFK as president and Robert Kennedy as attorney general, and a book signed by Jacqueline Kennedy are among the items in a series of JFK artifacts on the website.

GettyImages-3321635 1962: US statesman John F Kennedy, 35th president of the USA, making a speech. Central Press/Getty Images