The Great British Bake Off host pulled an on-air prank as he appeared on the BBC evening show on Thursday.
Alex revealed that prior to coming on air, Matt had told her and Jermaine about his part in the hit police drama, and asked him to fill viewers in.
“Even my family don’t know this,” Matt began. “It happened four months ago and it’s amazing how they kept it a secret, but I appear in two episodes of Line Of Duty in this series. I play a special policeman.”
As Alex then revealed how Matt had not seen any footage from his appearance, Matt continued his tall story, saying “No, and they only give you some of the scripts, so I don’t even know what happens in the end.”
Alex then introduced what she thought was a clip of Matt in action in the show, but turned out to be old footage with Matt edited into an interview with Superintendent Ted Hastings.
“I’ve got a question for you,” the Little Britain star tells Hastings. “What on earth is going on in this show? I watched it last week and I couldn’t follow it.”
Holding a plastic police helmet, he adds: “I’m not wearing this, it looks very cheap.”
As the show cut back to the studio, Alex and Jermaine were mortified to have fallen for the prank.
“Oh my god! You’re joking! Wow. I completely fell for it hook, line and sinker,” Alex said.
Jermaine added: “I was thinking what role is he playing here? How are they going to fit him in?”
Matt joked: “They are going to cast someone with this face in Line Of Duty? They’d be ruining it.”
The current series of Line Of Duty kicked off last month and sees AC-12 taking on their most enigmatic adversary yet as they investigate the conduct of DCI Joanne Davidson, played by Kelly Macdonald.
The One Show airs weekdays at 7pm on BBC One, while Line Of Duty airs Sundays at 9pm on BBC One.