Sarah Jessica Parker kept Cynthia Nixon's gubernatorial bid secret for a month

Sarah Jessica Parker says her "Sex and The City" co-star, Cynthia Nixon had confided in her about her decision to contest for New York governor post a month before she made the official announcement.

The 53-year-old actor praised Nixon, saying she is a "formidable" and "brave" candidate for the office, EW reported.

"She shared with me that she was going to announce about a month before. I was incredibly excited and proud of her. I think she's already been good for the conversation. This is what's exciting about primaries, right? People can inspire an incumbent to reconsider policy... Or just have a robust conversation.

"She's formidable. She's incredibly bright. She's really brave. She's excited about this race, and it's challenging. But, she seems enormously energised and I think it's good for us to have a competitor - it's always good. A challenger is good," Parker said.

She was speaking at a retrospective session on her career at the 2018 Tribeca Film Festival.

Parker had earlier released a short, straight statement after Nixon's announcement in which she almost stopped short of an endorsement for her co-actor, saying "Cynthia has been my friend and colleague since we were little girls. I look forward to talking to her about her New York state gubernatorial bid."

The 51-year-old actor will be up against incumbent Governor Andrew Cuomo, who is running for his third term, in the Democratic primary to be held on September 13.

If elected, she would create history by becoming the first female and openly gay governor of New York.

Parker added Nixon has always been inclined towards activism but she did not anticipate her taking the gubernatorial plunge.

"I didn't (anticipate) that she would seek a gubernatorial seat. She's been an activist her entire life. She loves this city, she's been involved in conversations about policies that have been really important to her for many years: education, public schools, housing... The larger issues that are part of conversations that affect us here (in the city) and upstate as well," she said.