The pair called it quits after Summer Monteys-Fullam, 24, refused to sign a non-disclosure agreement.
Last week, Paul, 53, addressed the break-up on Instagram, saying he had “never understood anyone who makes money selling stories and courting the paps”.
On Saturday, Summer responded on the social media platform.
She said: “I am very disappointed and distressed by false and highly defamatory statements that Paul has made about me online since our break-up.
“I have had no alternative but to instruct my solicitors Brett Wilson LLP to pursue legal proceedings against Paul in order to obtain vindication.
“I have never courted publicity over my relationship with Paul and I am deeply disappointed that our relationship has ended in this way.”
She previously said she felt “betrayed” when asked to sign the non-disclosure agreement “as I had clearly demonstrated loyalty to Paul during our relationship”.
A solicitor for Paul previously confirmed he had asked her to sign such an agreement.
It comes after a judge last month signalled the end of his marriage to wife Alexandra, who had filed for divorce on the grounds that the celebrity baker had committed adultery.
District Judge Robert Duddridge held that the star had committed adultery and that Alexandra found it “intolerable” to live with him.
He concluded that the marriage had broken down irretrievably and granted a decree nisi.
A marriage does not formally end until a decree absolute has been granted.
The couple married in 1998 and split two years ago.