Meghan and Harry's cute birthday message to nephew Prince Louis

The Duke and Duchess of Sussex are set to become parents any day now, but for the time being they're happily enjoying their roles as aunty and uncle.

On Tuesday, their youngest nephew Prince Louis of Cambridge turned one, an occasion marked by the release of three seriously cute portraits of the little royal.

When the photos—captured by Catherine, Duchess of Cambridge—were shared on the Kensington Palace Instagram page, they were quickly inundated with swooning comments from fans.

However, among the thousands of messages was one from an Instagram handle any fan of the British monarchy would immediately recognise: Sussex Royal.

"Happy birthday Louis!" Harry and Meghan wrote under one of the portraits, adding, "Sending lots of love from both of us xo."

(For good measure, they also threw in a cake and balloon emoji.)

Naturally, the post gained plenty of attention from fellow users, many clearly enjoying the personal tone of the message.

"We can't wait for yours to be born and then the four cousins to play together!" one wrote in response to the couple.

Some fans speculated the message had been written by Meghan on behalf of herself and her husband, adding to the popular theory the Duchess runs the Sussex Royal account.

However, because apparently even innocent birthday wishes can't go unscrutinised on social media, others were critical of the couple for calling the toddler by his first name and not his full title.

LISTEN: The Windsors podcast takes a closer look at the life of Meghan Markle. (Post continues.)

"Just Louis? It must be Prince Louis," one commenter declared.

"This is not a private account hence the need to address the gorgeous Prince Louis accordingly, with respect," another suggested.

While the Sussexes might have ordinarily popped over to chez Cambridge to wish their nephew a happy birthday in person, their recent move to Windsor makes that a little tricky.

Before they relocated, Harry and Meghan had been living mere metres from the Duke and Duchess of Cambridge in the grounds of Kensington Palace, giving them plenty of quality time with Louis and his older siblings, Prince George and Princess Charlotte.

The couple's first child is due any day now; the Duchess is believed to be due in late April or early May.