BBC drama SS-GB shed more than two million viewers in its second outing, losing more than a third of the people who watched its debut.
The series, which is adapted from Len Deighton’s 1978 novel of the same name and looks at what would have happened if the Nazis had won the Battle of Britain, drew 6.1 million viewers last week.
But Sunday night’s episode on BBC One saw the ratings plummet to 3.9 million viewers.
The first instalment of the five-part series, which stars Kate Bosworth and Sam Riley, was criticised over its sound quality, with viewers complaining that some of the actors mumbled.
A few dubbed it the “slurred Reich”.
The sound concerns appeared to have eased following the second episode, although some people said on social media that they were still finding it difficult to hear and had switched on the subtitles.
The programme itself also received mixed reviews on Twitter.
One person wrote: “I thought last week’s episode was dull due to bad sound.
“Watching it with subtitles this week & realised it is just dull & boring.”
Another said: “Gave it another go but SS-GB really is remarkably dull. Such a shame, it could have been so good!”