The Scottish actor, who starred in Trainspotting and Moulin Rouge, will be joined by footballers Robin Van Persie and Alex Oxlade-Chamberlain, who are ambassadors for BT Sport.
Two new ads will air during the Gogglebox break tonight. They are the first in a new campaign called 'Behind the Scenes', which shows the process of making an ad, including celebrity castings, rehearsals, pre-production meetings, post-production process and voice over recordings.
This will replace the existing TV campaign that features young flatmates.
Over the next few months, "behind the scenes" will feature a variety of famous actors, celebrities and sports personalities who will be the voice of reason against characters including the BT clients, the director and the ad agency creative.
The ads were created by Abbott Mead Vickers BBDO. Maxus bought the media.
The first spot, titled "big stunt", promotes BT TV. It features a motorcyclist smashing through a set while reciting the benefits of BT TV.
McGregor speaks to the BT clients and questions whether the ad actually works.
McGregor will also star in a second ad in the break, called "silence", in which he sits idly at a laptop. When he questions the team behind the ad, he is told that doing nothing reflects the ad agency creative’s vision of the BT Infinity Hub 5, which does everything for you.
McGregor said: "I’m thrilled to be starring in this new campaign for BT. I hope the viewing public enjoy the action, alongside the laughs."
The TV campaign will be accompanied by an outdoor ad campaign that will run during April and digital and social media activity to introduce the new characters.
BT’s consumer marketing director, David James, said: "BT is very excited to be launching its new marketing campaign. It follows our previous hugely popular campaigns, including Adam and Jane’s love story as well as the flatmates.
"The public got to know these characters and took them into their hearts. We are confident that they will do the same with our new host of intriguing characters.
"We have stuck with the humorous nature of our previous marketing campaigns which have proved so popular."
The ads were created by AMV BBDO creatives Richard Peretti and Gary Lathwell, and were directed by Randy Krallman through Smuggler.