Archie Mitchell was murdered in the Queen Vic on Christmas Day
Some 16.6 million viewers tuned in to Friday's EastEnders episode to see who murdered Archie Mitchell, the BBC says.
The killer was finally revealed in the 30-minute episode, which was filmed and broadcast live to mark the BBC One soap's 25th anniversary.
It featured 51 members of the cast, who had been rehearsing for three days.
In the finale, Stacey Slater, played by actress Lacey Turner, admitted she was Archie's murderer - the culmination of a long-running plotline.
Archie, who had amassed a string of enemies during his brief time on the show and had raped Stacey, was bludgeoned with a bust of Queen Victoria in the Queen Vic pub on Christmas Day.
Turner said did not know she would be unmasked as the killer until 20 minutes before the live transmission.
To be honest it still hasn't sunk in and I can't believe that I was the one
The live episode also featured the departure of Bradley Branning, played by Charlie Clements, who fell to his death off a building in Albert Square. He and Stacey had got married for the second time in the previous episode.
And speaking after the show, Turner said she was surprised to hear about her character's role in the big drama.
She said: "It's funny as usually you are told when you get such a big storyline but this really must be the best kept secret in EastEnders history as we have all been trying to guess for weeks now...
"To be honest it still hasn't sunk in and I can't believe that I was the one."
Turner lost her voice ahead of filming, prompting speculation in the tabloid newspapers earlier in the week that she might not even be well enough to appear in the episode.
But, despite having no knowledge of her pivotal role, Turner was determined to appear.
"I just made sure that I rested my voice throughout the day so I could save it," she said.
The viewing figures for the live episode peaked at 16.6 million and averaged 15.6 million. A further 1.4 million people tuned in to watch the repeat on BBC Three later on in the evening.
The BBC said 4.3 million viewers turned over to BBC Three after the live show to see EastEnders Live: The aftermath, which featured interviews with the cast and crew. During the show, Larry Lamb, who played Archie, admitted that he had himself thought the murderer would be revealed as his on-screen daughter Ronnie, played by Samantha Womack.
The BBC Three viewing figure was a record for a non-terrestrial channel, the BBC said.
The figures are "overnight" ratings which do not take into account those viewers who may have recorded the programme to watch at a later time or those who chose to catch the episode via BBC iPlayer.
Prior to the live episode, EastEnders' New Year's Day show - featuring the Masood wedding - had been the most-watched TV show of 2010 with a peak audience of 12.3 million.
But the figures are a far cry from the record-breaking 30.15 million viewers who watched the 1986 Christmas Day episode in which Dirty Den handed Angie divorce papers.
Diederick Santer, EastEnders executive producer said: "These numbers go far beyond what we ever hoped for.
"I am so proud of our cast and crew for their incredible performance last night. This rating is the icing on our silver anniversary cake."