Lucy Beale's killer will be revealed live on air during a special week of EastEnders episodes to mark the soap's 30th anniversary.
It means show bosses can keep the murderer's identity a secret from cast and crew until the last minute before they are unmasked in February.
The anniversary of the BBC soap will include one 30-minute live episode and “live elements” in the other episodes.
BBC One controller Charlotte Moore said: “BBC One will bring the nation together to celebrate 30 years of EastEnders by going live across the anniversary week. Next February will mark a massive event on the channel by creating the ultimate 'doof doof' and finally reveal who killed Lucy Beale.”
Viewers have watched as suspicion has fallen on several different characters since Lucy's (Hetti Bywater) murder in April, with the identity of the killer still a mystery.
The long-running soap marked its 25th anniversary in 2009 with its first ever live episode when it was revealed Stacey Branning (Lacey Turner) had murdered Archie Mitchell (Larry Lamb).
BBC drama boss Ben Stephenson said: “After a captivating year of drama in 2014, EastEnders will top this in February 2015 with the most ambitious anniversary any soap has attempted. Live week allows us to keep loads of secrets from the cast, crew and the audience until the very last minute. With shocking reveals and unguessable twists it's going to be a week that will change Albert Square forever.”
The live week starts on Monday 16 February.