Mackenzie Crook's sister-in-law Laurel Aldridge, 62, hasn't been seen since Tuesday.
The police issued an update on Saturday urging anyone to come forward if they have any information relating to Laurel's whereabouts.
Sussex Police tweeted: "We're continuing to be concerned for the welfare of Laurel Aldridge, 62, from Walberton. She's vulnerable and hasn’t been seen since Tuesday."
Police have urged people to check their outbuildings for The Office star’s sister-in-law, who has gone missing in West Sussex.
The British actor, who is also known for roles in Pirates Of The Caribbean and Worzel Gummidge, said the family were “obviously really worried”.
Sussex Police said it was “continuing to be concerned” about Laurel Aldridge, who has been missing from the Walberton area, near Arundel, since Tuesday.
Laurel, is considered to be “very vulnerable” and was last seen leaving her home wearing a turquoise fleece, a maroon tartan scarf and brown hat and had a grey puffer jacket.
She is described as being around 5’4” with grey/blonde highlighted hair and sometimes wears glasses.
Detective Sergeant Alan Fenn said: “We would like to thank the public for the information which has been provided to us thus far.
“Our officers are pursuing multiple lines of inquiry and are determined to find Laurel. We would be grateful if residents in the Walberton and Slindon areas could check their outbuildings for any sightings of Laurel.
“Also, anyone who was in the Walberton area on Tuesday morning or has video footage of someone matching Laurel’s description is asked to report it to us. The family are being supported by a dedicated officer and being updated with the investigation.”
Speaking to ITV, The Office actor said the disappearance of his sister-in-law, who is undergoing chemotherapy, was “very out of character”.
“She left her house early Tuesday morning and she’s not been seen since,” he told ITV.
“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 she might have taken rest.
“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.”
People are asked to report any information to police by calling 101 if it is non-urgent or 999 if it is urgent quoting serial number 347 of 14/02.
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