Royal Tour of Canada
Kate Middleton and Princess Charlotte Chris Jackson/ Getty Images

Kate Middleton may want to watch out, because her 16-month old daughter Princess Charlotte is already threatening to steal her crown as the most popular British royal judging by memorabilia sales.

According to Royal Collection Enterprises, which sells official royal momentoes, a range of commemorative china created for the toddler's birthday in May 2015 has sold out.

There has also been an additional surge in sales at the palace shops and online due to Charlotte's great grandmother Queen Elizabeth II celebrating her 90th birthday this year.

The Charlotte china range has sold out online and there are only a few items left in stock at the shops located in and around Buckingham Palace and Windsor Castle, according to People.

A spokeswoman for the collection said: "Retail sales bounced back," after an official report from Royal Collection Enterprises found an increased turnover increased of more than 11.2% for online sales from April 2015 to March 2016.

She added: "Merchandise relating to The Queen's longest reign milestone in September 2015, and the 90th birthday celebrations in 2016 both helped to drive sale volumes.

Royal Tour of Canada
The Duchess of Cambridge plays with her children, while Prince William blows up balloons Chris Jackson/ Getty Images

Visitor numbers across Buckingham Palace, Windsor Castle, Clarence House and Holyroodhouse and four other venues were also up to 2.5 million, a figure in which she dubbed a "slight improvement."

The range of official chinaware was produced by the Royal Collection Trust to celebrate the birth of HRH Princess Charlotte of Cambridge and the exclusively designed range includes items such as a coffee mug (19.95), a pillbox (£29), a tankard (£39) and plate (£45).

The beautiful designs of the commemorative china pieces are adorned with the lion and unicorn from the Royal Arms in the style figures from a carousel, along with the coronet of the Duke and Duchess of Cambridge and a pattern of oak leaves from the Middleton family's coat of arms. Each piece is gilded by hand with 22-carat gold and bears the words: "Celebrating our new royal baby". Star and tassel motifs were previously seen on the china to celebrate HRH Prince George of Cambridge's birth.