Pippa Middleton's father-in-law is under investigation over the suspected rape of a minor.

The father-in-law of Pippa Middleton, whose sister Kate is married to Britain’s Prince William, has been placed under formal investigation over suspected rape of a minor.

David Matthews, who is the father of Pippa Middleton’s husband James Matthews, was arrested on Tuesday by the Juvenile Protection Brigade (BPM) and formally put under investigation for suspected rape of a minor under his authority, a court source says.

Paris prosecutors arrested Matthews during a visit to France, and later released him and placed him under judicial control, the source said. The source did not say when he was released. French police can hold suspects 24 or 48 hours in such cases.

The source said the alleged rape took place in 1998-99. Europe 1 reported that a complaint was filed in 2017.

David and Jane Matthews attend the wedding of their son James. Image: Getty.

Reuters could not immediately reach Matthews nor any spokespeople or lawyers for him.

Being placed under judicial control means that prosecutors have attached certain conditions to his release or imposed certain limits on who he can meet or where he can go. The source did not say what conditions had been attached in Matthews' case.

A spokesperson for the family said Mr Matthews, who is the owner of the prestigious five-star Eden Rock Caribbean hotel in St Barths along with wife Jane Matthews, denies the allegations.

"David Matthews categorically denies the allegation and unequivocally contests the untrue and scandalous accusation."

Pippa Middleton came to national attention in Britain as the maid of honour at her sister's royal wedding to William in 2011.

Her own lavish wedding to James Matthews last May was one of the most widely reported social events of the year, attended by William and his brother Harry, grandsons of Queen Elizabeth.