The BBC TV drama Jamaica Inn lost a quarter of its audience after sound issues left viewers barely able to understand the dialogue.
The audience for the second episode of the adaptation of Daphne du Maurier's novel fell by 1.6 million following hundreds of viewer complaints over mumbling in the first episode.
The BBC's controller of drama commissioning has apologised after complaints about inaudible dialogue in the programme rose to nearly 800 by yesterday morning, including 252 made following the second episode on Tuesday night.
Ben Stephenson said the corporation was looking into the sound issues that plagued the first episode, aired on BBC1 on Monday.
Stephenson said: "If no one can understand what they're saying, then there's a problem. I think actors not being clear is one part of it, but my understanding about the complaints about Jamaica Inn was more complex than that, so I think it's probably not right to just single out that.
"Of course we want [actors] to give brilliant performances and you've got to respect that, but if no one can understand what they're saying, then there's a problem." .
While some viewers reported an improvement of the sound quality in the second episode, the cast's strong West Country accents also caused difficulties and others complained they still had to use subtitles to understand the actors.
Before the screening of the second episode, the programme's writer, Emma Frost, asked viewers to spare a thought for the sound operator, Matt Gill, adding: "Be nice to him. He's crying."
Gill also responded to the complaints on Twitter, saying: "I don't think anyone 'let' it go out like that. Technical faults happen … Best guess is a transmission problem as it sounds fine on previews, episode two broadcast and iPlayer."
This was echoed by Kirsty Lang, the presenter of the BBC Radio 4 arts programme Front Row, who tweeted: "I didn't have any problem hearing #JamaicaInn when I was shown a preview so it must have been a fault in transmission."
The first episode of the gothic drama pulled in 6.1 million viewers on Easter Monday but that number dropped to only 4.5 million for the second episode, prompting fears that the audience numbers could decline even further for Wednesday's finale.
Set in 1821, the three-part adaptation stars the former Downton Abbey actor Jessica Brown Findlay and tells the story of Mary Yellan, an orphan who becomes involved with Cornish smugglers.
Sound technicians at the BBC have reacted angrily to the suggestion that they were at fault for the incoherent dialogue.
Ian Sands, of the Broadcasting, Entertainment, Cinematograph and Theatre Union (BECTU), said that the performances in Jamaica Inn had been regarded as challenging by those recording the drama.
He added: "In this instance the general understanding among my colleagues is that this is an artistic issue. Low-level mumbled lines are not a technical issue, they are an artistic issue."
Speaking to Radio 4, Lorraine Heggessey, the former controller of BBC1, said: "It is a bit of a mystery how it got onto air with such bad sound quality and it did spoil what could have been a really beautiful drama. I just wonder whether they went for authenticity over clarity."
• This article was amended on 24 April 2014 to correct the name of the Broadcasting, Entertainment, Cinematograph and Theatre Union, from Broadcasting, Entertainment & Cinema Trades union as the original said.
