Mackenzie Crook said his family are 'clutching at straws' to find his missing sister-in-law, as he gives an update.
Lauren Aldridge, 62, went missing in West Sussex on Valentine's Day.
She has been missing from the Walberton area since Tuesday 14 February.
Speaking on Good Morning Britain, Mackenzie said: "It’s a week now, it was this time last Tuesday that she left the house with nothing but the clothes she was wearing, no keys, money, phone, and she’s not been seen since.
"I’m here at Slindon cricket ground which was the last positive sighting we had of her and that was a week ago, since then, nothing.
"She was was diagnosed with cancer last year and she missed her fifth chemo session last Tuesday and we think that’s what has triggered some sort of crisis in her and she’s gone off for a long walk somewhere…we are really clutching at straws now after a week."
Sussex Police have asked the public to check their outbuildings and believe Laurel may have crossed the A27 and A29 roads, and want drivers to check for dashcam footage of her there.
Mackenzie, who is best known for his acting career in the likes of The Office and Pirates of the Caribbean, has been open with the public about the situation.
Previously speaking to ITV, he said this is "very out of character" for Laurel.
He 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."
Mackenzie added: "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."
Laurel was last seen leaving her home wearing a turquoise fleece, tartan scarf and a brown hat, with her carrying a grey puffer jacket.
She has been described as being around 5ft 4in tall and has grey/blonde hair and is sometimes wearing glasses.
Those with any information on the whereabouts of Laurel are being asked to call 101 if it isn't urgent or 999 if it is urgent, quoting serial number 347.
