The Marvel film is the most anticipated of 2019, and is expected to be one of the biggest movies of all time when it opens next week.
Fans’ fears that plot details would be spoiled for them before they saw the end product are par for the course with these franchise blockbusters, but it’s an even bigger concern now after footage leaked online overnight.
The clip – thought to be from the final act of the film – emerged on social media before being swiftly taken down.
Now, the Russo Brothers have issued a plea to fans to keep the big spoilers to themselves.
In a letter published on Twitter, they said: “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 all 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.”
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They added: “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 others, the same way you wouldn’t want it spoiled for you.
“Remember, Thanos still demands your silence.
“As always, good luck and happy viewing. The Russo Brothers.”
Meanwhile, moderators on Reddit have issued a warning to fans to keep discussion on spoilers restricted to only the specifically designated threads.
New TV spots are continuing to be released, including one that could confirm a previous theory about Captain Marvel.
Another takes fans right back through the preceding movies in the Infinity Saga.
Avengers: Endgame is out on April 25 in the UK.