BBC1's singing contest slumped to an average of 6.6 million viewers, a drop from an average of 7.2 million viewers who tuned in to watch the 2013 final, won by Northern Irish singer Andrea Begley.
The final of this year's third series pulled in the fewest viewers for a final in the history of the programme.
The final of the first series had an average of 7.1 million viewers. This year's final was won by 19-year-old Jermain Jackman.
Meanwhile, judge Kylie Minogue has said she is still unsure whether she will rejoin the show for the next series.