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A copy of the film recorded in a Chinese cinema began circulating online between 4pm and 5pm China time on Wednesday, and has since spread to English networks, according to TorrentFreak.
A source told the site that the copy is of terrible quality.
They said: "The image flickers constantly throughout.
"It's semi-rotated and is littered throughout with watermarks for a gambling site that spin around the screen."
Understandably - given the devout nature of Marvel fans and the immense amount of time and money invested in the film - directors Joe and Anthony Russo have been at pains to encourage people not to spoil the movie for others.
They recently penned a letter to fans, pleading with them not give away spoilers.
The letter read: "To the greatest fans in the world.
"This is it. This is the end. The end of an unprecedented narrative mosaic spanning eleven years and eleven franchises.
"For all of you who have been on this journey with us since the very beginning, sharing every high and low with your family, your friends, your classmates, your co-workers. Investing so deeply in every character and storyline. Laughing. Cheering Shedding tears.
"Giving so freely of your thoughts and emotions in spirited dialogue, theories, fan art, and fan fiction. Please know that the two of us, along with everyone involved in Endgame, have worked tirelessly for the last three years with the sole intention of delivering a surprising and emotionally powerful conclusion to the Infinity Saga.
It continued: "Because so many of you have invested your time, your hearts, and your souls into these stories, we're once again asking for your help.
"When you see Endgame in the coming weeks, please don't spoil it for other, the same way you wouldn't want it spoiled for you."
They concluded by saying: "Remember, Thanos still demands your silence. As always, good luck and happy viewing..."
Inevitably, it is now a dangerous period for Marvel fans who haven't seen the film, as spoilers are going to inescapably increase with frequency, be it via mean-spirited pricks or absent-minded slips of the tongue.
Apparently, it's really bloody good, so time to batten down the hatches and go offline 'til you've seen for yourself.
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