Fans post plot spoilers and screengrabs after season six opener makes its way on to the internet ahead of official broadcast...

It was one of the most highly anticipated returns in television history – but fans of Game of Thrones were let in on the plot sooner than expected after the latest episode was mistakenly made available online hours before the premiere.

The first episode of season six of the fantasy series, based on George RR Martin’s books, was broadcast simultaneously on Sky Atlantic at 2am in the UK and on HBO in the US. But before it aired, fans posted plot spoilers on websites including Reddit and the Internet Movie Database (IMDb) after footage made its way online early, according to the Telegraph.

While some websites claim HBO was responsible for the leak, others have blamed a Canadian streaming website. The spoilers, some of which were backed up with alleged screengrabs and accompanied by French subtitles, have since been taken down.

Game Of Thrones stars British actors including Emilia Clarke, Lena Headey and Kit Harington, whose future on the show has been the subject of much speculation after his character Jon Snow was stabbed at the end of the last series. The question of whether he was really dead was debated on social media after the episode, which was called The Red Woman.

On Twitter, #GameOfThrones was the top trending hashtag globally for hours before and after the show aired.

As the first reviews were published, the Independent said the premiere “wasn’t a hugely memorable episode, mostly being given over to setting up the playing field for the rest of the season”.

US magazine Forbes said the opening episode was a “strong, surprising return to Game of Thrones”, adding that readers of the books must resign themselves to the “vast differences between TV show and book at this point”.

Time magazine said the long-awaited episode “did more work tying up loose narrative threads than it did giving us new provocations”, while the Hollywood Reporter said the show “did what it needed to” do by “putting this mammoth locomotive back on the track”.

The latest series is the first time GoT has moved past the books’ plot. HBO has announced the show will return for a seventh series in 2017.