After turning heads at her sister Kates wedding to Prince William, Pippa Middleton was Saturday graduating from bridesmaid to bride at what the British media were dubbing the society wedding of the decade.

The 33-year-old Pippa made global headlines with a figure-hugging outfit when Kate, now the Duchess of Cambridge, married William, second in line to the throne, in 2011.

On Saturday, she was on her way to exchange vows with wealthy financier James Matthews, 41, at a lavish wedding where William and Kate's young children, Prince George and Princess Charlotte, are expected to play a starring role.

The ceremony is taking place at St Mark's Church in Englefield, a quiet village in the Berkshire countryside west of London, near the Middleton family home.

The event will reportedly feature a flypast by a World War II-era Spitfire plane and a party afterwards in a £100,000 ($130,000, 115,000 euro) glass marquee.

The presence of senior royals means security is expected to be tight, and no-fly zones banning aircraft including drones have been imposed around the area.

William and Kate's three-year-old son Prince George -- Queen Elizabeth II's great-grandson and third in line to the throne, was to be a page boy.

Princess Charlotte, two, will be a bridesmaid.

William's younger brother Prince Harry is also expected to attend the wedding, likely with his US actress girlfriend Meghan Markle.

Harry's love life is a source of much speculation, prompting him last year to issue a rare warning to the media not to harass Markle.

'Startling' global recognition

Pippa Middleton rocketed into the public eye at her sister's wedding, which had an estimated worldwide television audience of two billion.

Pictures of her standing behind the royal bride in a white Alexander McQueen maid of honour's dress filled newspapers for weeks afterwards.

"It is a bit startling to achieve global recognition (if that's the right word) before the age of 30 on account of your sister, your brother-in-law and your bottom," she once wrote.

She is the second of three Middleton children, a year younger than Kate and four years older than businessman James.

A socialite on London's posh party circuit, she is a regular guest in the royal box at the Wimbledon tennis championships.

She became engaged to Matthews, a former professional racing driver and heir to the Scottish title of Laird of Glen Affric, on July 17 last year.

Middleton graduated from Edinburgh University with an English literature degree, before working part-time for her parents' party paraphernalia business.

She has published two books -- a 2012 guide to party planning that was widely mocked for offering glaringly obvious advice, and a healthy recipes charity book. She also had a column for a supermarket magazine.