ITV has said the Good Morning Britain studios were evacuated this morning "as a precaution".
"It was an isolated incident which was quickly resolved," the broadcaster said in a statement.
Viewers at home were left watching a pre-recorded interview when the fire alarm went off mid-broadcast.
Presenters Ben Shephard and Susanna Reid were interviewing Emmerdale actors Charley Webb and Anthony Quinlan via a live link from Yorkshire when the alarm at the London Television Centre went off.
Reid said: "Terribly sorry."
Shephard added: "We have a little bit of drama in the studio. I don't know whether the Emmerdale scriptwriters have done something, but the fire alarm appears to be going off in the studio which has never, ever happened before."
Reid said: "I'm asking the director - does this mean we have to evacuate the studio? I think if you hear a fire alarm and you're told to evacuate the studio, it's probably a good idea to do so.
"I'm going to check we have a little bit of stand-by tape to run. If we do that."
Viewers could hear the automated voice on the alarm, saying: "Leave immediately by the nearest exit."
Reid said to the Emmerdale stars: "Thanks so much for talking to us, because we're leaving the studio and that means we're going to leave you with a little bit of what we call in the business stand-by tape."
The show switched to a pre-recorded 30-year-special with Lorraine Kelly and Shephard.
A message appeared on TV screens in the top right-hand corner, reading: "Sorry for the interruption to our normal programme we'll be back to the Lorraine studio as soon as possible."
Reid tweeted as soon as they returned, posting a photo of herself and Shephard with the caption: "Back in the building - all fine, only thing lost in the fire alert seems to be Ben's tie."
A member of staff at the building, who did not want to be named, said a "fuse box overheated and went on fire on the third floor".
Good Morning Britain is filmed on the third floor, he added.