Bradley Cooper was "horrified" when Jessica Chastain's grandmother sat in his lap.

The 44-year-old actress was hosting a party at her home with both the Silver Linings Playbook actor and her gran Marilyn Herst among the guests, and the Molly's Game star admitted her elderly relative quickly proved she had no inhibitions despite being amongst a star-studded crowd.

Whilst appearing on The Ellen DeGeneres Show, Jessica said: “A few years ago I had a party at my house and Bradley Cooper was there. And my grandmother is now at the age where she just really doesn't care, you know? She's just like, "I'm gonna just do whatever I want."'

So in the middle of the party she walked over to Bradley and just sat on his lap.

“He was horrified actually! He had never met her. He didn't know who she was.”

Jessica, whose new movie, the spy thriller The 355 opens this week in the US,had to reassure an alarmed Bradley, 47, that it was her grandma.

"I saw it happening in slow motion and I was like noooo, and I just started going 'it's my grandma, it's my grandma, it's my grandma, and he just started going 'Okay! Okay! Hi, Grandma!'."

Jessica admitted Bradley isn't the only Hollywood hunk to have caught her grandma's eye.

She said: “I worked with Al Pacino — she was very excited about that, I took a picture of the two of them together and it was on her bedside table — I worked with Matthew McConaughey, she was stoked.”

And the Interstellar star has been trying to help Marilyn find a date.

She said: "I used to take pictures of her and put them on

“I’ve been trying to get her dates for a long time."

Jessica previously hailed her grandma as a "total catch". Speaking on The Graham Norton Show about Marilyn, she said: “She's an amazing person, but I still haven't found her a man.

“She's a total catch and I've been posting pictures of her exercising with me — she is very fit!”

The 355 opens in Singapore cinemas on Jan 13 while Bradley Cooper's Nightmare Alley opens exclusively in Cathay cinemas on Jan 27.  — BANG SHOWBIZ

Photo: TPG News/Click Photos