Mackenzie Crook has urged people to continue looking for his sister-in-law, Laurel Aldridge, who has been missing for three days.
The star of The Office, 51, has spoken of how the family are "really worried" for Ms Aldridge who has recently been receiving chemotherapy treatment.
It comes just days after Sussex Police also stated they were "concerned" about Laurel who was last seen in the Walberton area close to Arundel on Tuesday.
A spokesperson for the force said Ms Aldridge, 62, may have travelled as far north as Bignor Hill.
Mackenzie has now claimed that the disappearance of Laurel is "very out of character" in an emotional interview.
Speaking to ITV, the actor who has also appeared in Pirates Of The Caribbean said: "She left her house early Tuesday morning and she's not been seen since.
"That's in the Walberton area where a lot of people are looking in the local woodlands along the roads and stuff like that.
"I'd really like to appeal to people to go out into their back gardens and have a look in places where she might have taken rest."
He went on to add: "She's been missing out for three nights - we're entering the fourth one now so, we're obviously really worried.
"If I could appeal to everyone, even if they checked before, check again in likely places where she might have crept in to lay down for the night."
Mackenzie explained that when she was last seen, Laurel was wearing a bright blue fleece which had a grey coat over it as well as a brown woolly hat, a turtle scarf and black trousers.
Laurel who is known to take her dogs out for walks is said to have left her home with no keys and didn't take her pets or her car.
Her son-in-law Charlie Wheeler said: "She left the house at 7.20 yesterday [Tuesday] morning.
"We saw her on the Ring doorbell and all she had was warm clothes on her, but she didn't have any keys, she didn't have any cards, she didn't take the car, she didn't take her dogs and she's just vanished."
He added: "She's currently undergoing chemo treatment and we’re worried she might be having some sort of mental health episode
Taking to Twitter, Sussex police also issued a plea saying: "We're growing increasingly concerned for Laurel Aldridge, who has been missing from Walberton, near Arundel, since yesterday.
"She may have travelled as far north as Bignor Hill, and we're urging anyone who sees her to please call 999 quoting 347 of 14/02."
