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The younger sister of Catherine, the Duchess of Cambridge, was invited to a Gerpinnes property in west Belgium owned by billionaire steel magnate Baron Albert Frere, local newspaper La Capital reported.
The December 1 hunt, organised by the Baron's grandsons, shot six wild boar and nine deer, the report said.
Pippa, 29, was among the 20-strong party, some of who did not recognise the London party planner and author as she introduced herself as "Phillipa".
"It is sad to see someone in such an influential position indulging in this kind of needless cruelty for their own entertainment," League Against Cruel Sports spokesman Joe Duckworth told British tabloid the Daily Star.
Pippa's level of involvement in the hunt is unknown, but the brunette who shot to stardom after appearing as bridesmaid at her sister's royal wedding to Prince William, is reported by media to be a "keen shot" and has attended such events in the past.
In April 2012 it was a toy firearm that attracted unwanted attention for Pippa.
As she was pursued by paparazzi through the streets of Paris, the driver of a convertible in which she was a passenger pointed a handgun at a photographer.
Later identified as French aristocrat and fashion designer Viscount Arthur de Soultrait, the gunman apologised for the incident during which he said he held a toy gun.