* Baby George to be christened in small, private service
* William and wife Kate invite just 21 guests
* Princess Diana's friend among the seven godparents
Just 21 guests were invited to the christening of Britain's Prince George on Wednesday, a sign of how much his parents intend to guard the privacy of a baby born to be king.
Prince William, whose mother Diana was killed in a car crash in Paris in 1997, hounded by paparazzi, and his wife Kate only invited very close family members and godparents, according to a guest list released ahead of the 3 p.m. (1400 GMT) ceremony.
Queen Elizabeth, her husband Prince Philip, heir to the throne Prince Charles, his wife Camilla, and William's brother Harry will attend the service in which Archbishop of Canterbury Justin Welby will baptise the three-month old baby with water from the River Jordan.
Although the christening will be held behind closed doors, in the Chapel Royal of St. James's Palace, well-wishers gathered outside, hoping to see guests arriving at the 16th century building in central London commissioned by King Henry VIII.
Millions of people watched coverage of the couple in July as they left the hospital with their son, the only public sighting of George since his birth on July 22.
The parents named six friends and William's cousin Zara Phillips as godparents, a break with the tradition of choosing royal dignitaries to take on the ceremonial role, continuing their effort to portray a more informal, modern image to austerity-hit Britons.
Kate's parents, Michael and Carole Middleton, and her sister Pippa were due to be among the guests. A friend of Princess Diana, Julia Samuel, is one of the godmothers alongside Kate's schoolfriend Emilia Jardine-Paterson.
"This is not a state but a family occasion and I think the royal couple want to keep George out of the public eye as much as possible to give him a normal upbringing," said Clarissa Campbell, historian of monarchy at Anglia Ruskin University.
"It's also very much Her Majesty's wish that the royal family is not seen as an expensive institution in these days."
The palace has named Jason Bell, 44, best known for his portraits of rock stars and Hollywood actors, as the one photographer to record the event.
As well as the christening, Bell is expected to shoot the first portrait of four generations of the royal family in more than 100 years, with the queen and her three direct heirs, Charles, William and George.
A royal spokesman said George would be wearing a white satin robe that is a replica of one made in 1841 for the christening of Queen Victoria's eldest daughter. A tier of a cake made for Willam and Kate's 2011 wedding will be served at a private tea held after the christening.