Kate Middleton
The Duchess of Cambridge to appear in a documentary about Queen's life and reign Chris Jackson/ Reuters

Since her fairy-tale wedding to Prince William in 2011, Kate Middleton has lent her voice to a whole host of worthy causes and attended her fair share official engagements. And now, the Duchess of Cambridge has marked another milestone as a fully fledged royal. The 34-year-old has given her first solo television interview ahead of the Queen's upcoming 90th birthday.

The landmark moment is part of an ITV programme marking the Queen's life and reign. The mother-of-two joined elite members of the royal family in paying tribute to the monarch and is set to contribute to the film by also discussing the Queen's history.

During her years in the public eye, Middleton has had little to no experience on the small screen. In fact, the last time she was interviewed on television was in November 2010 when she and Prince William sat down with broadcaster Tom Bradby on the day their engagement was announced. Back then, she let her fiancé do most of the talking, but this time around – and with five years of royal duties under her belt – she is expected to give a more dynamic performance.

"We've been fortunate to have the participation of so many members of the royal family to mark this historic milestone in the life of the Queen," said Nick Kent, executive producer for Oxford Film and Television, said in a statement. "It's a remarkable story of an extraordinary life which in so many ways reflects the changing face of the nation.

The two-hour film, which has the working title The Queen At Ninety, is set to be broadcast in May, shortly after Her Majesty turns 90 on 21 April. Jo Clinton-Davis, ITV's controller of factual, who co-commissioned the programme, added: "As she reaches her 90th birthday, this film aims to offer a fresh insight into our monarch's life and work."

In January, it was announced that Middleton was set to become the new patron of the All England Lawn Tennis & Croquet Club as the Queen prepares to scale back her royal duties in a bid to ease her workload. "There have been discussions but the Queen is mindful that she does not want organisations with which she is associated to think that when she turns 90, that's it — it's goodbye. That will not be the case at all," a royal source said in a Sunday Times report.

Queen Elizabeth II
Queen Elizabeth II is the longest reigning monarch.IBTimes UK/Reuters