The unveiling of a secret Holmes sister was one of the biggest plot twists in the history of Sherlock – but fewer people than ever were watching.

For the BBC drama lost more than 2million viewers in the space of one week, slumping to its lowest overnight ratings of 6million on Sunday night.

Many fans have complained that the plot of the Benedict Cumberbatch-fronted series has become too clever, with many switching off as a result of the ‘over- the-top storyline’.

Others chose instead to watch ITV’s Endeavour, which emerged as strong competition in the same slot, with 4.5million viewers watching the first episode of series four.

Perhaps more worrying for the BBC than the competition from a rival broadcaster was the fact that Sherlock was not even its most-watched show of the day, as it was beaten by Countryfile and Antiques Roadshow.

The early-evening shows attracted 6.9million and 6.4million respectively, which are levels Sherlock would normally be expected to swamp.

Indeed, the New Year’s Day special in 2016 – which divided opinion – was watched by 8.4million and the three-part series in 2014 averaged 8.9million.

Last week's episode featured Sherlock tackling rich and powerful serial killer Culverton Smith, played by Toby Jones, pictured

Despite its poor ratings, Sunday’s episode was hailed by many fans as a ‘return to form’, with many viewers praising the shock revelation of Euros Holmes in the closing moments.

Such was the level of trickery that actress Sian Brooke had been disguised as three characters before she was finally revealed as the secret sister of Sherlock and Mycroft.

Acclaimed actor Toby Jones was also praised for his portrayal of high-flying entrepreneur Culverton Smith, who was later shown to be a serial killer.

Several viewers commented on a troubling resemblance between the villainous character and Jimmy Savile, with the former BBC presenter’s name quickly trending on social media.

It climaxed with a drug-addled Sherlock being attacked by Smith in a hospital the philanthropist used to hide his murders

One audience member posted: ‘Wasn’t convinced by last week’s outing but #Sherlock was fab tonight. Crooked-toothed Toby Jones channelling Jimmy Savile was proper creepy!’

Another wrote: ‘#Sherlock was awesome but I’m still not sure how I feel about the BBC rehashing its darkest hour for prime time entertainment... #savile.’

Despite low overnight ratings, it is expected that the second episode will be watched by a further 2million people via catch-up services.