Britain's Prince George celebrates his second birthday Wednesday, when the royals are planning a small family gathering for the future king with his baby sister Charlotte as he embarks on the so-called "terrible twos".

George, Charlotte and their parents Prince William and his wife Kate, the Duke and Duchess of Cambridge, are to spend the day together at their country home.

Queen Elizabeth II's great-grandson is directly in line to become king after his grandfather Prince Charles and then William, who did his first shift as an air ambulance pilot last week.

The Cambridges split their time between their Kensington Palace apartment in London and Anmer Hall, a country mansion on Queen Elizabeth's private Sandringham estate in Norfolk, eastern England.

"They will be spending the day privately together as a family," a Kensington Palace spokeswoman told AFP.

Regardless of William's shift patterns, "they will have the opportunity to spend some time together".

William will still be settling into his new four-days-on, four-days-off rotation, flying out of Cambridge Airport in eastern England.

Before his first shift on July 13, William spoke about the arrival of Charlotte -- and how George was becoming a handful.

"It's been fantastic, she's been a little joy of heaven," he said of Charlotte, who was born on May 2.

"At the same time it is more responsibility looking after two little ones, especially when George is around -- he's a little monkey."

- Rare public appearances -

 Prince George Alexander Louis of Cambridge was born at St Mary's Hospital in central London at 4:24 pm on July 22, 2013, weighing eight pounds and six ounces (3.8 kilogrammes).

Third in line to the throne, his birth produced a frenzy of excitement.

He made his first overseas tour to New Zealand and Australia in April 2014 and celebrated his first birthday with a private party at Kensington Palace.

George has made few public appearances so far, with his parents keen to protect him from the media spotlight.

His visit to the newborn Charlotte at St Mary's was the first time he had been seen at a public occasion in Britain since his own birth at the hospital.

Official photos have periodically been released to mark events such as his baptism.

He was last seen at Charlotte's christening, where he was spotted interacting with the Queen, peering into his sister's pram and waving to the crowds.

The youngster was wearing red shorts and a white embroidered shirt at the event and his brief appearances have not stopped him from becoming a retro fashion trend-setter.

The blond-haired prince often appears in outfits mirroring those worn by William as a toddler, and they typically sell out within hours.