Laurel Aldridge, 62, who is the older sister of the star's wife, was reported missing from her home in Walberton, in the Arundel area of West Sussex, on Valentine's Day, Tuesday 14 February.

She had been undergoing chemotherapy treatment, but did not turn up for a treatment session that day.

Crook, who is known for his roles in Detectorists, The Office, Pirates Of The Caribbean and Worzel Gummidge, said in a new interview that her disappearance has been "agonising" for the family but they have a "strong suspicion" she is still in the local area.

"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," he told Good Morning Britain.

The star's televised appeal came from Slindon cricket ground in West Sussex, which he said was the location for "the last positive sighting we had of her and that was a week ago, since then, nothing".

Crook said his family believe her disappearance could be linked to ongoing chemotherapy treatment.

"She 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," he said. "We are really clutching at straws now after a week."

'We are asking people to check all of those places in their garden'

The actor continued: "At the moment everyone is containing their emotions, I'm sure the time will come for that later but right now everyone is very determined and driven.

"We've got a strong suspicion that she is in the local area but that's just a hunch we're going on. We can't imagine that she has gone very far afield."

Sussex Police previously said officers may have information which could place Mrs Aldridge's last known location, and to reach this destination she may have crossed busy roads including the A27 and A29.

Crook said that having so many wooded areas nearby was making the search "quite overwhelming".

He continued: "We have been combing the woodlands for a week now, we can only think she has hopefully taken shelter in a back garden, in a shed, in an outhouse, that's what we're hoping.

"We are asking people to check all of those places in their garden - even if they've done it before, to go back and check again.

"Now we are also asking people to check dashcam [footage] because there are specific roads the police think she may have crossed in that time."

Detective Sergeant Alan Fenn has also urged residents in the Walberton and Slindon areas to "check their outbuildings for any sightings of Laurel".

Mrs Aldridge is described as being around 5ft 4in with grey/blonde highlighted hair. She sometimes wears glasses and was last seen wearing a turquoise fleece, a maroon tartan scarf and a brown hat. She also had a grey puffer jacket with her which she is likely to have been wearing.