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Kate started dating William in 2003, after meeting him at the University of St Andrews. Soon after, she met Harry.

He was still a teenager, about to leave Eton, and was deciding what to do with his life.

Harry revealed in his book Spare how he had worries about the princess taking his brother away from him.

He detailed how he ‘consoled’ himself by thinking about the fun they would all enjoy together in the future.

After an extended gap year, when he travelled to Australia and Lesotho, he finally went to Sandhurst in 2005.

A source at St Andrews told Daily Mail in 2016: ‘From the earliest days, Kate always maintained an open-door policy to Prince Harry.

‘While they were students she always made him feel welcome at the four-bedroom cottage they rented on distant royal cousin Henry Cheape’s estate, Strathtyrum, just outside St Andrews. The three also spent weekends together at the cottage, Tam-na-Ghar, on the Balmoral estate.’

Another insider said: ‘Cynics — the sort that said she deliberately swapped Edinburgh University for St Andrews when she found out William was going there — might say she knew that getting on with Harry was key to her relationship with William.’

‘Harry, although he can be moody, over-sensitive, and unreliable to his girlfriends, is otherwise enormously good fun. Everybody likes him. Why shouldn’t Kate, too?’

Years before William proposed, Kate was already exerting her benign, civilising influence on both brothers.

In London, while William was learning how to fly, he and Kate would retreat to the flat in Clarence House he shared with Harry.