Prince Harry and Meghan Markle attend Prince Louis' christening

Prince Harry and Meghan Markle played an important role at the christening of Prince Louis: that of doting uncle and aunty.

The Duke and Duchess of Sussex attended the baptism at St James's Palace in London on Monday, arriving alongside the newborn's proud parents, Prince William and Kate Middleton.

The famously tactile couple were affectionate as always, holding hands as they greeted Archbishop of Canterbury, Justin Welby, who baptised baby Louis.

WATCH: Meghan giggles with Archbishop of Canterbury, Justin Welby, as she arrives at Prince Louis' christening

The former Suits star wore an olive green Ralph Lauren dress for the occasion, topped with a hat designed by Stephen Jones.

RELATED: All the sweetest Royal Family moments from Prince Louis' christening

According to reports, Prince Harry marked the occasion of his nephew's christening with a special, not to mention rare, gift: a first-edition Winnie-the-Pooh book from 1926.

The 33-year-old reportedly forked out $AU14,000 on the A.A. Milne book - said to be the first in a library he is building for his two nephews and niece in memory of Princess Diana.

“She loved all the old classics and Harry had the brilliant idea of starting a little library of first editions for Louis, Charlotte and George to enjoy as they get older.”

While the couple's Windsor Castle wedding came just a few weeks after Prince Louis' birth, the newborn was presumably deemed too young to attend the event.

Though they have the honour of bring the prince's aunty and uncle, the Duke and Duchess aren't among Louis' six godparents.

This isn't likely to be a personal slight; rather, it reflects Prince William and Catherine's habit of choosing friends and relatives to be their children's godparents, rather than close family members.