The boys each picked something from their mum's collection (Image: Getty Images)

When Prince William got down on one knee to ask his long-term girlfriend to be his wife, he presented her with his late mother's engagement ring.

The sapphire and diamond band is one of the most famous pieces of jewellery in history, worn by both Princess Diana and Kate Middleton, who would become the Duchess of Cambridge.

But Kate wasn't always going to have the famous royal ring, and she only ended up wearing it thanks to a touching sacrifice from her future brother-in-law, Prince Harry.

After the death of their mother, Diana, in 1997 the boys were taken to choose a "keepsake" from her belongings in Kensington Palace, according to reports from a source close to the royals.

They said: "William picked his mum’s Cartier watch, and Harry picked her sapphire and diamond engagement ring."

 Kate now wears Diana's sapphire engagement ring (Image: AFP/Getty Images)

Harry was just 12 when he chose to keep his mum's engagement ring.

However, as William was the first to get engaged he gave it up so his older brother could offer it to his bride, Kate Middleton.

The source added: “They had an agreement that whoever got engaged first would have Diana’s ring – and of course that was William and Kate."

So when William popped the question in Africa in 2010, he used the family heirloom and Kate proudly showed it off in the couple's official engagement photos.

 Harry designed Meghan's engagement ring himself (Image: AFP via Getty Images)

But this of course meant that when Harry knew it was time to ask Meghan Markle to be his wife in 2017, he needed to find a ring.

While he didn't have one of his mum's old pieces, he made sure to include her in the stunning final design.

He designed the ring himself, using two diamonds from Diana's collection and one from Botswana, where he does a lot of charity work.

However Meghan decided to redesign the ring slightly, making it even more glitzy.

 He used two diamonds from Diana's collection (Image: PA)

Instead of the original thick gold band, the diamonds now sit on a much thinner micropavé band.

It's unclear when the work was done, however she was spotted without the ring during the last few months of her pregnancy so it could have been done then.

Both Kate and Meghan wear their engagement rings alongside Welsh gold wedding rings, as is traditional for royal women.