Battle of Brothers, a book written by historian Robert Lacey about the royal family and the relationship between the Dukes of Cambridge and Sussex, is due to be released later this month.

In the book, which is being serialised by the Daily Mail, Lacey claims that when the monarch first extended an invitation for Catherine to join the royal family for their traditional Christmas lunch in 2006, the then-24-year-old declined.

According to Lacey, who works as a historical consultant on Netflix's hit period drama The Crown, Catherine wanted to wait until she was engaged before joining in the royal family’s Christmas festivities.

Catherine “would go to Sandringham on Christmas Day only when she was engaged and had a ring to prove it”, the historian states in the book.

The Duke and Duchess of Cambridge met in 2001, when they were both studying at the University of St Andrews in Scotland.

After moving in together in 2002 with several other university friends, the pair made their first public appearance together in 2006, while attending the wedding of Laura Parker-Bowles, the daughter of Camilla, the Duchess of Cornwall.

After a brief split the following year, in November 2010 Catherine and Prince William announced their engagement, the future duchess wearing an oval sapphire engagement ring that previously belonged to Princess Diana.

The Duchess of Cambridge made her first appearance at Sandringham for the royal family’s Christmas celebrations in 2011, eight months after she and the Duke of Cambridge wed at Westminster Abbey.

It is widely regarded as tradition for partners of members of the royal family to only attend Christmas festivities at Sandringham once they are married.

However, in 2017 Meghan, the future Duchess of Sussex, joined her fiancé, Prince Harry, and the royal family at Sandringham for Christmas, five months before their wedding in Windsor.

The Independent has contacted Kensington and Buckingham Palace for comment.