Shackleton, who helped Paul McCartney in his split with Heather Mills, also represented Prince Charles in his split from Diana as well as Madonna in her divorce from Guy Ritchie, reported Sun online.
Nigella, 53, who has two children, started speaking to Shackleton shortly after learning that Saatchi, 70, had announced their divorce in a paper, blaming her for not defending him over pictures where he is seen grabbing her neck in a restaurant.
"Nigella never wanted it to come to this but Fiona has been a rock of support and she is glad to have her on her side. Nigella is soft so she needs someone strong representing her and they don't come any tougher than Fiona," a source close to Lawson said.
Shackleton is Lawson's cousin and is famous for her no-nonsense style. In 2008, she saved McCartney from a huge payout which enraged Mills so much that she threw water over Shackleton.
Together Lawson and Saatchi are worth 130 million pounds with Lawson's TV success accounting for a large slice of the property.