Kate and William were once left 'questioning their relationship' after heated argument

The Prince and Princess of Wales celebrated their 12th wedding anniversary earlier this year, and since they exchanged vows over a decade ago they have not only gone on to become a royal power couple, but have welcomed three children, Prince George, Princess Charlotte and Prince Louis.

But a few years before they walked down the aisle, the couple briefly split and took some time apart. 

William and Kate had been dating since their university days, with their romance eventually confirmed in 2004 when they were photographed enjoying a skiing holiday together. However, they separated in 2007 with William reportedly telling the Queen that he had second thoughts about their relationship. 

After a few months apart, they reunited during a trip to the Seychelles and made a 'marriage pact' and were engaged three years later.

While William and Kate addressed their brief split during their engagement announcement in 2010 - with Kate admitting she 'wasn't very happy about it' and William putting it down to being 'very young' - the separation was reportedly fuelled by the Prince's decision to forgo New Year's Eve with the Middleton family in 2006, according to the Mirror. 

William had promised to spend the celebratory evening with Kate as she wasn't invited to the royal Christmas lunch at Sandringham. At the time it was only offered to wives, so when he pulled out of their New Years plans they were left 'evaluating' their relationship. 

A colleague of Kate's told the Mirror at the time that they had witnessed the now-Princess pacing in the office car park during the intense phone call. 

They said: "It was clear it was William - and definitely a tiff. Around the middle of the day, she was pacing the car park outside the office on her mobile. A few people noticed because it's a pretty unusual thing to do. She'd deliberately gone outside to take the call because she didn't want to be overheard. 

"Kate was walking up and down and looking upset as if she was having an argument. But she didn't cry. Word went round that it was William and they were having problems. Now it's clear what it was all about. When she came back to the office she didn't say a thing and disappeared for the rest of the day.

"We didn't think anything about it at the time. But looking back, the call was definitely a girlfriend-boyfriend tiff."

Royal insiders have claimed the break was the 'best thing for them' and that their separation actually secured Kate's role as future Queen given how she 'gracefully' handled the break-up.