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As of yesterday (23 September), a total of 10.9 million people had watched the first episode since it aired on Monday 14 September, according to the station.
This figure takes into account the live audience and repeat showings, as well as viewers on ITV Hub and on non-TV devices.
The data means the true-crime series has recorded the channel's biggest launch since Wild At Heart in 2006. It has also overtaken Quiz as the most-watched drama on ITV Hub, raking in more than 1.7 million viewers - 300,000 more than Quiz, which was released in April.
The three-part series sees David Tennant play Dennis Nilsen, one of Britain's most infamous serial killers, while Line of Duty's Daniel Mays plays Detective Chief Inspector Peter Jay.
Nilsen, who died on 12 May 2018 aged 72, was a serial killer and necrophile who is believed to have brutally murdered at least 12 young men and boys between 1978 and 1983.
The Scottish murderer was arrested by police at 23 Cranley Gardens, Muswell Hill, London, on 9 February 1983 after a plumber responded to complaints made by Nilsen and other tenants about the drains. After opening the drain cover, the plumber discovered a flesh-like substance and a number of bones.
Speaking about the killer, Tennant recently told LADbible he felt it was 'right and proper' that Nilsen died before the series aired.
He said: "After he was arrested, Dennis Nilsen became obsessed with the legend of 'Des' - the reputation that he left behind. Whenever he slipped out of public consciousness, there was almost a sense that he wanted to get back into it. That's why I'm relieved he's not alive.
"I would hate for this to go out and for him to be sitting in some cell somewhere imaging we were in any way glorifying him. I'm sure he would have complained about what we said and everything we did. At the same time, he would have been rather smugly pleased he was on television.
"I think it's right and proper it's transmitting after he's gone."
Tennant has been widely praised by viewers and critics alike for his unsettling portrayal of the killer.
Taking to social media, one person wrote: "Can we hand David Tennant all the awards that exist Brilliant #Des."
Another said: "If David Tennant doesn't win all the awards for this, something is very wrong."
A third added: "Please just give David Tennant the NTA, the BAFTA, the Oscar and any other award which involves acting, because he deserves them all #Des."
You can watch the series now on the ITV Hub.
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