By Mike Collett-White

LONDON (Reuters) - A set of pearls once belonging to Marie Antoinette and taken to Britain by a friend for safekeeping will go on sale in December, and are expected to fetch up to $800,000.

Now part of a diamond, ruby and pearl necklace, France's last queen gave a bag of pearls and diamonds to Lady Sutherland, the British ambassador's wife, before she fled revolutionary France in 1792, a year before Marie Antoinette's death.

"Lady Sutherland was wife of the ambassador and friends with the queen, and they had children of the same age," said Raymond Sancroft-Baker, senior director of Christie's jewelry in London.

"When you are in a dire situation, there are not many people you can trust and the key was to give the jewels to someone with diplomatic immunity," he told Reuters.

Marie Antoinette, legendary for her extravagance, did not know her fate at the time, he said, and would have hoped to be reunited with her treasures one day.

"Hope springs eternal," added Sancroft-Baker.

According to Christie's, Sutherland arranged for clothes and linen to be sent to the queen while she was in prison.

"This was reportedly the last gesture of kindness shown to the doomed queen," the auctioneer said in a statement.

Marie Antoinette was executed by guillotine in October 1793.

The diamonds were made into a necklace, while the pearls were mounted later for the occasion of the marriage of Sutherland's grandson in 1849.

Christie's did not specify which of Sutherland's descendants was selling the necklace.

"The owner said it just sits in the bank the whole time, and there comes a time for everything," Sancroft-Baker said.

He hoped the pearls, which have never been offered at auction before and remained in the same family for over 200 years, would be made available for the public to see.

"The Louvre might be interested, for example," he said, adding that the story behind the necklace made it one of the most important sales he had overseen at Christie's.

"It's right up there in the top 10 we've ever sold, because its provenance is rock solid, as far as we can be aware. There are documents to go with it and contemporary supporting evidence."

The necklace will go under the hammer at the Magnificent Jewelry sale in London on December 12, and is expected to make between 350,000 and 400,000 pounds ($700-800,000).