According to friends, the younger sister of the Duchess of Cambridge has been delaying the news out of respect for her sister Kate, who gave birth last month to her and Prince William's first baby, Prince George.
A close friend of the couple said in July: "An engagement is definitely on the cards."
Pippa, 29, and her handsome boyfriend Nico, have openly shown affection for one another since bringing their relationship into the public eye at the beginning of the year.
The eligible brunette is reported to have told friends that her stockbroker beau is "the one".
The party planner-turned-columnist was beaming as she strolled hand-in-hand with Nico in London's swish Chelsea neighbourhood on Friday night. She looked stunning in a short skirt and white long-sleeved top with strappy heels and a clutch.
Kate's younger sister was first linked to Nico at the start of the year — and by February the 35-year-old had already been introduced to her family.
He was invited into the Middleton fold when he joined Pippa, Kate and Prince William on holiday in paradise island Mustique, along with her parents Michael and Carole and her brother James.
Nico's delighted mother has also given the relationship her blessing, describing Pippa as "absolutely charming".
While Pippa has been linked to aristocracy in the past, dating suitors including investment banker James Matthews and financier and former England cricketer Alex Loudon, Nico has a distinctly humble background.
He grew up in a middle-class red-brick terraced house in Folkestone, Kent, and attended the Harvey Grammar School in Folkestone, before earning a business degree at Kingston University. He then went on to make his millions as a stockbroker at Deutsche Bank.