Royal fans are convinced this clip of the Cambridges at play reveals Prince William's affectionate nickname for his daughter Charlotte.

The video, released by Kensington Palace on Instagram, shows the royal father-of-three with his wife Kate, 37, and their three children in the Duchess' Back To Nature garden at the Chelsea Flower Show last Sunday.

While most of the clip is subtitled, there is one short section of William speaking to Charlotte while on the rope swing that is missed out.

In new photos released by the Duke and Duchess of Cambridge, the couple's three children play in the Back to Nature Garden. Photo / AP.
In new photos released by the Duke and Duchess of Cambridge, the couple's three children play in the Back to Nature Garden. Photo / AP.

Dozens of social media users took to the comment section to say they think William calls his daughter "mignonette". The word is a diminutive form of the French "mignon", meaning "cute".

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However others believe the Duke of Cambridge, 36, simply asks: "Have you been on this yet?"

Dozens of social media users took to the comment section to say they think William calls his daughter mignonette. Photo / AP.
Dozens of social media users took to the comment section to say they think William calls his daughter mignonette. Photo / AP.

One Twitter user asked: "Anyone notice right before he asked Charlotte to push him, he called her by a lovely nickname. Someone from KP IG's (Kensington Palace Instagram's) comment section suggested William called her 'Mignonette' which means 'delicate'."

Another tweeted: "Mignonette I think - cutie. Not 100 per cent though."

Though one tweeter was less convinced, claiming the prince simply asked: "Have you been on here?"

And one Instagram user wrote: "I hear the end of the question 'Have you been on it?' With us hearing the 'been on it' part. That would also make sense, given that he then asks for a push. (Charlotte would rather run off to do her own thing, of course.)"

It wouldn't be the first time the Cambridges have used nicknames to refer to each other.

Back in January, Prince George revealed his nickname is "Archie" when he struck up a conversation with a stranger on a walk with his grandmother Carole Middleton in Berkshire.

READ MORE: Royal day out: Prince Louis takes first steps on camera

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The 5-year-old started talking to the woman while stroking her dog after playing with his younger sister, Charlotte, in a stream.

His revelation sparked rumours earlier this month that Prince Harry and Meghan Markle were inspired to use the moniker for their son, Archie Harrison.

Duchess Kate designed the garden with the Royal Horticultural Society. Photo / AP.
Duchess Kate designed the garden with the Royal Horticultural Society. Photo / AP.