Giles, who designed model Abbey Clancy's wedding dress, was pictured leaving Pippa's west London home with a large black garment bag, prompting speculation that the pair were meeting about her wedding.
An assistant who was also carrying a second dress bag was snapped at the property. Pippa, 33, and her mother Carole, 61, emerged shortly after.
While Hello!'s exciting discovery may suggest that Giles has been lined up to create Pippa's dream gown, a source has told usthat the philanthropist is still deciding.
"Like her sister, Pippa is keen to go for a British designer and is seeing a few potentials before she makes the big decision," the source told Hello!.
Pippa's older sister Kate tied the knot with Prince William in a stunning ivory gown designed by Sarah Burton, the creative director of British luxury fashion house Alexander McQueen.
Giles Deacon is known for his colourful and playful style. He was employed by the fashion houses Bottega Veneta and Gucci before establishing his own label.
Fashion experts have been sharing their suggestions for what Pippa may wear on the big day.
David Emanuel, who co-designed Princess Diana's gown, said: "I personally would put her in something very soft, perhaps chiffon, with a train of course because it's dramatic. Possibly a little lace bodice, soft and fluid. I mean, she's got a great shape."
"And I would say the complete opposite of Kate just so she makes her own statement really," he added to Mail Online. "Soft and pretty."
Pippa and James announced their engagement on July 19. The hedge fund manager popped the question with an exquisite four-carat asscher-cut ring, said to be worth £200,000.
The couple are expected to say "I do" early next year.