Bookmaker William Hill has slashed its odds on Sean being the murderer to 4/7 following a flurry of bets from punters.
The troubled character's strong showing in the betting comes despite the fact that he has been off screen since January 2009 following Rob Kazinsky's decision to quit the role.
Meanwhile, Janine Butcher's love interest Ryan Malloy - played by Neil McDermott - remains second favourite in the betting at 11/2.
Jack Branning (Scott Maslen) and Walford legend Peggy Mitchell (Barbara Windsor) are joint third favourites with odds of 9/1.
In a statement, William Hill spokesman Rupert Adams said over 5,000 bets have been received in the past 24 hours and that a total of £1 million could be gambled on the outcome by the time the market closes.
He added: "The BBC have done a brilliant job keeping everyone guessing as we have taken bets on 24 different members of the cast, but the absent Sean Slater has been today's big gamble."
The killer's identity will finally be revealed to viewers in tomorrow night's live 25th anniversary episode.