This is despite the series six finale being largely panned by fans and critics, who slammed the "disappointing" revelation that the elusive Fourth Man was actually DCI Ian Buckells.
Creator Jed Mercurio has been forced to defend how he concluded the show, which has not yet confirmed another series, while many viewers have also praised the finale.
Meanwhile, stars Adrian Dunbar, Martin Compston, and Vicky McClure have revealed they promised they would get "AC-12million" tattoos if they hit the milestone of 12million viewers - which they succeeded by far.
The viewing figures are 80,000 higher than the previous record, which was set by an episode of Heartbeat on ITV in February 2000 which had a TV audience of 15.16 million.
This means the BBC One police thriller is now the highest-rated TV drama series since March 1999, when an episode of ITV’s A Touch Of Frost won an audience of 15.83 million.
Only a tiny number of programmes have managed to attract TV audiences of more than 15 million in the past decade, according to analysis by the PA news agency.
Line Of Duty now shares this honour with The Great British Bake Off (15.9 million in October 2016), EastEnders (16.4 million in February 2010), Gavin & Stacey (17.1 million in December 2019) and The X Factor (17.7 million in December 2010).
They are consolidated ratings, meaning they include people who recorded and watched a broadcast up to seven days later – the industry standard for measuring TV audiences.
The list includes plenty of household favourites, such as Inspector Morse and Doctor Who, though five of the 10 date from either 2000 or 2001.
Here is the top 10 in full, compiled by the PA news agency using Barb data. It includes only drama series, not soap operas, and is based on the highest-rated instance of each programme:
Line Of Duty, May 2 2021 (BBC1): 15.24m
Heartbeat, February 6 2000 (ITV): 15.16m
A Touch Of Frost, January 14 2001 (ITV): 14.69m
Bodyguard, September 23 2018 (BBC1): 14.34m
Inspector Morse, November 15 2000 (ITV): 13.66m
Doctor Who, December 25 2007 (BBC1): 13.31m
Sherlock, January 1 2014 (BBC1): 12.72m
Downton Abbey, November 6 2011 (ITV): 12.45m
Auf Wiedersehen, Pet, April 28 2002 (BBC1): 12.41m
The Royal, January 19 2003 (ITV): 11.96m
* Line of Duty is available to stream on BBC iPlayer