As previously reported, Jessica has already arrived in London to help Meghan prepare for her big day. It is believed she was there when Meghan found her dress, and has spent lots of time with Meghan's groom, Harry.
Jessica, who is married to Ben Mulroney, son of former Canadian prime minister Brian Mulroney and one of Canada's top television presenters, has been friends with Meghan for years.
'Jessica helped Meghan with her wardrobe and introduced her to the right people,' a friend close to the pair said. 'Meghan adores Jessica, not just as a friend but also as someone whose taste and style are impeccable. Meghan looks up to Jessica and has, to a certain degree, modelled herself on her.'
Jessica and her husband Ben share three children - twins Brian and John, and daughter Ivy.
A royal source said: 'Jessica is Meghan's BFF and has been a huge influence. Meghan's turned to her for advice on everything from the dress, the flowers, decorations in church, the wedding lunch — and even the seating plan.
'Although Jess lives in Toronto, the two are on the phone all the time. Meg wanted Jess at the final fitting as it'll be the most famous dress of the year and she's quite nervous about it. It all went really well and they were delighted with the result.'