"The letters are being sold by a descendant of Pamela Plowden," Mark James, a Christie's expert said on Tuesday.
The most valuable letter, dated 19 July 1942 and seven pages long, is expected to fetch between 25,000 ($43,000 dollars) and 35,000 pounds.
"This is a very intimate and long letter showing two levels of interest, a public one and a personal one," James said.
"It is a fascinating combination as you discover a private Churchill being very close to Pamela and a public Churchill as prime minister writing about the war and its victims," he said.
James said the letters would be sold individually and not in lots and potential buyers, who have expressed some interest until now, had been individuals.
But for Alan Packwood, head of the Churchill Archive at Cambridge University, north of London, the sale of the letters is heartbreaking.
"Unfortunately, the archives don't have budget to purchase the letters"
Head of the Churchill Archive
"Unfortunately, the archives don't have budget to purchase the letters," Packwood said.
"We're certainly interested to learn about these letters, of course."
"Obviously, it would be a hope that they're kept together as a
group and that they could be somewhere where they could be looked at or if we could make a copy," he said.
Packwood said it would be even more interesting to have the letters Pamela wrote to Churchill, which he claimed to have destroyed and have never been found.
The letters span 63 years that begin with passionate love letters from the future prime minister, then mature into an
affectionate correspondence between friends before, during and after the war.
Churchill met Plowden, a renowned society beauty in Hyderabad, India when he was in his early twenties, and it is thought that they were informally engaged.
But just two years later she married Victor, Earl of Lytton, the son of a Viceroy of India.