New figures show the New Year's Day edition - the first episode in the latest three-part run - piled on an extra 3.5 million viewers who watched the show within seven days of broadcast after recording it or using timeshift viewing.
The episode featured consulting detective Sherlock Holmes surprising his sidekick John Watson by revealing he was alive after apparently falling to his death two years earlier.
The new totals are revealed in "consolidated" figures which combine the audience on the night it was broadcast with the numbers who watched in the following week. They reveal that the edition of Sherlock has succeeded the Christmas 2011 edition of ITV's Downton Abbey - with 3.2 million - as the biggest timeshift success.
Sherlock's figures - a combined 12.7 million - do not include those who have watched the programme on the BBC's own iPlayer service.
Charlotte Moore, the controller of BBC1, said: "I'm very proud to have Sherlock on BBC1 and see it topping the charts as the biggest consolidated audience ever. It demonstrates that ambition and quality matter to our audience."
Producer Sue Vertue, the wife of writer Steven Moffat, said: "When we began Sherlock, and it was an instant hit, we thought it couldn't get better. But each series has outdone the last and this is our biggest rating yet. Trying to believe this is really happening is a job in itself."
The BBC released new stills from the last edition of this series, His Last Vow, to be screened on Sunday evening in which Holmes (Benedict Cumberbatch) and Watson (Martin Freeman) are pictured with new villain Charles Augustus Magnussen (played by Danish actor Lars Mikkelsen).