Britain's Princess Charlotte has been christened in an intimate ceremony in the royal baby's second public appearance since her birth nine weeks ago.

Just 21 guests attended the low-key baptism held inside the 16th century Church of St Mary Magdalene on the country estate of the baby's great-grandmother Queen Elizabeth II on Sunday, as crowds waited outside.

LISTEN: Above, UK correspondent Gavin Grey talks to Rachel Smalley on KPMG Early Edition about Princess Charlotte's christening.

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Overnight campers and early risers, many wearing Union Jack-printed clothing, had braved wet weather to get a prime spot on the paddock outside the church at Sandringham estate in Norfolk, eastern England, ahead afternoon service.

Parents William and Kate walked to the service from a nearby royal residence with Charlotte and her brother George, giving the public their second glimpse of the little princess since she appeared on the steps of a London hospital after her birth on May 2.

Charlotte arrived for the ceremony in a vintage pram used by Queen Elizabeth for her own children in the past, pushed by her mother Kate, who wore an elegant cream outfit and hat.

Archbishop of Canterbury Justin Welby, the spiritual head of the world's Anglicans, performed the baptism, using the ornate silver gilt Lily Font that is part of the crown jewels and pouring holy water from the River Jordan on the baby's head.

The fourth-in-line to the throne wore a replica of the lace and satin christening gown made for Queen Victoria's eldest daughter in 1841.

As well as three future kings - Charles, William and George - other guests included the Queen and her husband Prince Philip, plus Kate's parents, Carole and Michael Middleton.

William and Kate announced early on Sunday that Charlotte would have five godparents, including William's cousin Laura Fellowes and Kate's cousin Adam Middleton, as well as Kate's friend Sophie Carter, and two close friends of William's, James Meade and Thomas van Straubenzee.

Afterwards guests enjoyed tea and cake at Sandringham House, the queen's nearby country retreat, including slices of christening cake - cut from a tier of William and Kate's 2011 wedding cake.