LONDON (REUTERS) - Prince Harry's fiancee, American actress Meghan Markle, will join Queen Elizabeth and other senior Windsors for their family Christmas celebrations in what commentators said was a break with royal protocol.

Last month, Queen Elizabeth's grandson, the fifth-in-line to the British throne, and Markle, who stars in the US TV legal drama Suits, announced their engagement with the marriage to take place at Windsor Castle in May next year.

The queen and her close family usually spend the festive season at her Sandringham estate in eastern England and Buckingham Palace said she and the other senior royals would attend the service at the local church on Christmas Day as they have done for many years.

However, in a break with tradition by which royal partners must wait until they are married before being invited, Markle would also be joining Harry's elder brother William and his wife Kate, the prince's office Kensington Palace said.

"You can expect to see the Duke and Duchess of Cambridge, Prince Harry and Ms Markle at Sandringham on Christmas Day," a spokesman said on Wednesday (Dec 13).

British media said it was unprecedented for an unmarried partner to join the royals at Christmas and that Kate was not invited to Sandringham after their engagement was announced in 2010.

However, William also did not attend as he was working as an air ambulance pilot in Wales at the time.