Is Sherlock over or will there be a fifth season?

Brace yourselves Sherlock fans: writer Mark Gattis has hinted that series four of the BBC1 drama might be the last.

Benedict Cumberbatch and Martin Freeman, who play Holmes and Doctor Watson, are apparently proving hard to pin down because of their demanding filming schedules.

"I honestly don't know if there will be any more. It's incredibly difficult to get Benedict and Martin's diaries to align," he told The Sun. "And obviously we left it in a very happy place… if that's the end I'd be very happy where we left it."

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Critics and viewers had detected a feeling of finality in the fourth series, which aired last month.

The last episode, entitled The Final Problem, brought several storylines to a "neat, almost elegiac conclusion", said Caroline Frost at the Huffington Post.

Sherlock and his brother Mycroft (played by Gatiss) found a new sibling connection, Moriarty was consigned to history and Watson forgave Sherlock for his wife's death. "There is nothing else to be done here," said Frost.

Last month, creator Steven Moffat said the finale was just the end of "stage one" in their "grand origin story" and that a new series might return to Holmes and Dr Watson when they are older, the ages originally portrayed in Arthur Conan Doyle's books.

"Normally, we present Sherlock Holmes and Dr Watson as in their 40s or 50s, and we started with them in their 30s so, in a way, it's the end of their youth," he told Vulture.

Bringing hope to fans, Moffat dismissed worries that there were no cliffhanging items left over after series four.

"There isn't something we have to come back to address," he said. "Which is quite good for us. Because if we do come back, we can just start with a knock on the door and a new client, and they can go and investigate."

Sherlock: Was final episode of season four all too much?

20 January

The Final Problem, the season four finale of BBC1's Sherlock, was a galloping, brain-scrambling romp that left some viewers wondering if the series had lost the plot and others eager for more.

For the past three weeks, series creators Mark Gatiss and Steven Moffat have been warping viewers' minds with a series of riddles, red herrings and revelations. But The Final Problem pulled out all stops to dazzle and bemuse, before ending on a seemingly sombre note.

Exploring why Sherlock is the way he his, the episode delved into a much-trailed childhood incident in which his apparent memories of his dog called Redbeard turned out to involve the death of his best friend, a young boy who was drowned at the hands of his deranged sister Eurus.

The revelations came thick and fast, as Eurus manipulated the guards in her island prison to do her evil bidding, killed a bunch of people and left Dr Watson to drown in a well – all while seemingly sending a jet plane hurtling towards a city.

However, Sherlock realised all his sister really needed was love and saved the day – his best friend Watson with it.

But was it all too much?

Andrew Blair at Den of Geek has previously questioned whether Sherlock's third season "jumped the shark", arguing the show breaks the fourth wall with asides and in-jokes that could be off-putting for the casual viewer.

Last night's episode went even further, according to some viewers on Twitter.