Marvel fans, be warned - footage spoiling major plot points for the Avengers: Endgame movie, set to hit cinemas here next Wednesday, was leaked yesterday afternoon and has been circulating online.

A 4½ minute montage, purportedly containing pivotal scenes from the final act of the hugely anticipated blockbuster, surfaced on various social media platforms including Twitter, Facebook and YouTube, though many links have since been taken down.

Short clips, screenshots and posts about the leak have also been posted online, dealing a blow to Disney, which owns the Marvel franchise and has taken pains to keep the three-hour film shrouded in secrecy.

The leak sent fans into an online fury, with many issuing warnings for others not to share spoilers or to refrain from accessing social media. They also shared ways to block words and hashtags that could potentially lead to spoilers.

Mr Reno Tan, 48, founder of western movie and pop culture community Movie Mania, told The New Paper that spoilers bother fans because most want to be surprised in the cinema watching it for the first time.

Mr Tan, who has not seen the leaked footage, said just hearing about it has dented the mood.

The avid fan said: "For many years, I have been watching premieres early and we never spoil it for people. But some may want to show off, gain attention."

Film-maker Jaze Phua, 27, who drew plaudits from the Avengers cast and crew for re-creating the movie's trailer, said the leak will not stop him from using social media, but he will be evading any whiff of a spoiler.

"As a fan, you will not click on the leaked footage under any circumstances... I will block whatever page that tries to show me spoilers," he said.

WHERE'S THE LEAK?

It is not clear where the leaked footage came from. Journalists and critics have not been allowed to see the film ahead of its premiere, with only 10 minutes of footage shown to critics in the US earlier this month and around 20 minutes shown to a select audience in Seoul this week. The cast has also been tight-lipped while doing press for the movie.

This is not the first time that a major blockbuster has leaked before its release. X-Men Origins: Wolverine, Ang Lee's Hulk and Star Wars: Episode III - Revenge of the Sith have suffered similar fates.

Nanyang Technological University assistant professor Liew Kai Khiun, who studies pop culture, said such leaks have, historically, rarely been critical blows because people go to the movies not just to see the story unfold but for the experience of witnessing it on the big screen.

He said: "There have always been leaks everywhere but how receptive people are to leaks, that is another thing... Tickets have already been bought, so I don't think financially it will affect the movie itself."

Pinsent Masons MPillay's Bryan Tan, an intellectual property and entertainment law expert, said tracking the source of the leak can be difficult and time-consuming, so the first step is usually to get distribution channels like social media platforms to take the leak down, reducing access.

"Just imagine if (the leak) makes the movie look bad, then it could have implications on how the movie performs, even if it is untrue," Mr Tan said, though he agreed this leak is unlikely to impact Endgame's box-office takings.

TNP has reached out to Disney for comment.

Meanwhile, Endgame directors Anthony and Joe Russo have posted an open letter on social media platforms under the hashtag #DontSpoilTheEndgame pleading fans not to spoil the movie.

"Because so man y of you have invested your time, your hearts, and your souls into these stories, we're once again asking for your help," the brothers wrote.

"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."

The letter ended with "remember, Thanos still demands your silence".