Princess Diana and Sarah Ferguson earned themselves the 'Merry Wives of Windsor' nickname during the early days of their marriages as they were such close friends.

Both women were in their twenties when they were thrust into royal life and the glare of the public spotlight, and the former Duchess of York and the Princess of Wales shared an unmistakable connection during the early years of their respective marriages.

The two even went on holiday together and were famously photographed at Ascot prodding Fergie's schoolfriend Lulu in the bottom with their umbrellas.

Fergie has repeatedly referred to Diana as her "best friend" and has spoken at length about her heartbreak following her tragic death, a loss made all the more agonising given that the pair were no longer on speaking terms at the time.

Sarah Ferguson never had the opportunity to make amends with her dear friend.

Their falling-out was addressed in Fergie's candid memoir, "Finding Sarah".

"Diana was one of the quickest wits I knew; nobody made me laugh like she did," she wrote.

"We took vacations together with our children. Sadly, at the end [of the Princess's life] we hadn't spoken for a year, although I never knew the reason, except that once Diana got something in her head it stuck there for a while."

However, the reason is believed to be very well known, the Mail Online reports.

Not only did Fergie purportedly breach her pledge to her friend not to discuss her in a previous book, My Story: by Sarah, the Duchess of York, but she also featured a particularly unflattering story about her.

The Duchess was quoted as saying she contracted a verruca after wearing a pair of shoes she borrowed from Diana.

An furious Diana severed all communication with her, and despite Fergie's efforts to reconcile, she never spoke to her again.