The Gavin And Stacey special has secured the highest overnight Christmas ratings in 12 years.
The episode was watched by an average of 11.6 million viewers, making it the biggest festive special since Christmas Day 2008.
It secured nearly half (49.2%) of the total viewing audience during its slot, according to the BBC.
The special picked up 10 years after the previous instalment and revealed that Gavin and Stacey (Mathew Horne and Joanna Page) now have three children, while Smithy and Nessa (James Corden and Ruth Jones) were successfully co-parenting their son, Neil.
It ended on a dramatic cliffhanger, with a marriage proposal between two major characters left unanswered.
The BBC also won the Christmas Day soap showdown after EastEnders, which aired at 9.30pm, secured the largest number of viewers with an average of 5.5 million.
On ITV, Coronation Street, on at 8pm, was watched by an average of 4.5 million, while Emmerdale, which aired earlier at 7pm, secured an average of 4.1 million.
BBC One claimed eight of the 10 most popular shows across Christmas Day.
The corporation's Strictly Come Dancing Christmas special, which was won by Debbie McGee, widow of magician Paul Daniels, earned an average of 5.5 million viewers, while the Queen's Christmas broadcast secured an average of 6.4 million.
Director of BBC content Charlotte Moore said: "Christmas Day on BBC One brought the nation together and entertained them in their millions, with the much-anticipated return of Gavin And Stacey taking the top spot in 2019, and making it the biggest in a decade.
"We delivered something for everyone with the seven most-popular programmes that cap off an incredible year for BBC One celebrating British talent and creativity."