Lee Mack stepped in to replace an unwell Bob Mortimer on this Sunday's episode of Mortimer & Whitehouse: Gone Fishing, British Comedy Guide can exclusively reveal, following Mortimer's hospitalisation.

However, Mortimer, who began the hit BBC Two series as part of his recovery from a triple heart bypass operation eight years ago, after his diagnosis with coronary heart disease, does make an appearance in the episode set on the tidal outcrop of Burgh Island on the south coast of Devon, via Facetime.

He and Whitehouse will also be shooting their fourth Christmas episode later this year, though the location has yet to be determined.

The penultimate episode of the current sixth series opens with Whitehouse learning the "big surprise" of his friend's illness.

Mortimer had previously revealed he'd been hospitalised during filming on Richard Herring's RHLSTP podcast last year.

He told Herring, who had joined them in Series 5 of Gone Fishing to discuss their various health issues by the River Thames, that: "I did a show last week, a fishing show, and there was only two and a half days filming and I did it Tuesday, Wednesday and half of Thursday and I was in hospital on the Saturday."

Mortimer also disclosed that his rheumatoid arthritis had returned, 29 years after it disappeared.

"It is really sad for me to know whether it will go. Yes I might be fat but actually, I am on steroids".

Mack, who also guest stars in tonight's episode of the sitcom Brassic, appeared alongside Whitehouse and Steve Coogan in Windermere in May to protest about the amount of sewage being discharged into England's largest lake.

Surprising Whitehouse in this weekend's episode, Mack joins him to fish for wrasse off Burgh Island and they enjoy a vegan lunch, before the pair leave to spend the night in a beautiful beach house where Agatha Christie wrote several of her novels. The following day, Whitehouse catches up with Mortimer and their faithful canine companion Ted.

Airing on Christmas Eve, last year's Christmas special attracted four million viewers and was the first episode to be extended to an hour, as well as the first not filmed in the UK, as Mortimer and Whitehouse sought cod and halibut off the Vesterålen and Lofoten Islands of Norway. A seventh series of the show has already been commissioned for next year.