The accounting firm responsible for the counting and collation of votes for the Oscars has apologised for its part in the blunder that saw La La Land announced as the winner of best picture instead of Moonlight.
The mix-up saw the musical announced as the victor due to duplicate sets of cards, held by the only two people who knew the results, being mistakenly handed out to presenters Warren Beatty and Faye Dunaway.
A statement from accountancy firm PricewaterhouseCoopers (PWC) said: " We sincerely apologise to Moonlight, La La Land, Warren Beatty, Faye Dunaway, and Oscar viewers for the error that was made during the award announcement for best picture.
READ: Major gaffe as wrong film named best picture at Academy Awards
"The presenters had mistakenly been given the wrong category envelope and when discovered, was immediately corrected.
"We are currently investigating how this could have happened, and deeply regret that this occurred.
"We appreciate the grace with which the nominees, the Academy, ABC, and Jimmy Kimmel handled the situation."
Beatty and Dunaway had been given a card that revealed that La La Land's Emma Stone was the leading actress winner, prompting them to incorrectly announce the musical as the winner.
Backstage, a confused Stone later said that she was still holding her winner's card in her hand when best picture was announced, but there are in fact two identical sets of cards.
Brian Cullinan and Martha Ruiz from PWC, the tax partner to the awards ceremony, were forced to intervene on stage amidst the confusion and chaos.
The major blunder came at the very end of the Oscars' ceremony, which was the worst for British talent in a decade.
The producers of La La Land were coming to the end of their acceptance speeches before it was revealed there had been a mix-up and that Moonlight was the victor of the coveted accolade.
La La Land producer Jordan Horowitz announced the error to the audience, holding up the correct card with the best picture winner, as he said: "There's a mistake. Moonlight you guys won best picture.
"This is no joke. I'm afraid they read the wrong thing. This is not a joke, Moonlight, you won best picture."
Beatty returned to the microphone to say: "I want to tell you what happened, I opened the envelope and it said Emma Stone, La La Land, and that is why I took such a long look at Faye and at you.
"I wasn't trying to be funny. This is Moonlight for best picture."
Cullinan and Ruiz are responsible for manually counting the 7,000 votes made by members of the Academy of Motion Picture Arts and Sciences and are the only two people to know the full list of winners before the ceremony.
They were earlier seen arriving on the red carpet ahead of the ceremony at the Dolby Theatre in Los Angeles, showcasing their matching briefcases filled with the 24 winners cards, which they each give out at either side of the stage to the presenters of the awards.
Moonlight's director Barry Jenkins said, after the film was correctly revealed as the winner: "Very clearly even in my dreams this can't be true. But to hell with dreams because this is true. It's true, it's not fake."
Meanwhile, while in recent years the Oscars has seen a spate of British actors winning the top awards, Sunday's ceremony was the worst for Britain's stars in 10 years.
White Helmets, a 40-minute Netflix film about a volunteer rescue group operating in war-torn Syria, saw its British director Orlando von Einsiedel and British producer Joanna Natasegara take the best documentary short award.
They were the only British winners at the prize-giving ceremony, as acting nominees including Dev Patel and Naomie Harris missed out.
La La Land, which had scored a record-tying 14 nominations, ended up with just six wins, including best actress for Emma Stone and best director for Damien Chazelle, who is the youngest winner of the prize at 32.
Moonlight, which follows the life of a young black boy dealing with his sexuality, scooped three prizes in total, landing best picture, best adapted screenplay and best supporting actor for Mahershala Ali.
The film's producer Dede Gardner said: "I'm still not sure this is real. It's very humbling to be up here and I hope it's inspiring to little black boys and brown girls who feel marginalised. I hope they take some inspiration from seeing this beautiful group of artists."
Host Jimmy Kimmel joked that he knew he would mess up compering the ceremony.
He added: "Personally I blame Steve Harvey for this," referring to Harvey's mistake in announcing the Miss Universe winner in 2015.
Stone, 28, who has already received a Golden Globe and Bafta for her role in the hit musical, thanked Chazelle and Ryan Gosling, saying of her co-star: "Thank you for making me laugh and for always raising the bar and for being the best partner in this crazy adventure."
Ali won the first Oscar of the 89th Academy Awards when he scooped the best supporting actor prize.
He welcomed his first child, Bari Najma, on February 22 and thanked his wife Amatus Sami-Karim saying: "My wife was in her third trimester doing awards seasons, we just had a daughter four days ago. I just want to thank her for being such a soldier through this process."
Viola Davis received a standing ovation and cheers as she won the best supporting actress Oscar for her role in Fences.
Davis, who has been nominated twice before, for Doubt and The Help, had been the frontrunner for the prize all through awards season.
She paid tribute to her co-star and director Denzel Washington, saying: "Oh captain, my captain, Denzel Washington, thank you for putting two entities in the driving seat, August Wilson and God and they served me well."
Casey Affleck won the best actor in a leading role Oscar for his role in Manchester By The Sea, beating fellow actors Ryan Gosling, Denzel Washington, Andrew Garfield and Viggo Mortensen.
Affleck, 41, was hugged and kissed by his brother, Batman star Ben Affleck, as his name was called and received a standing ovation as he made his way to the stage.
The actor was tearful as he made his acceptance speech, saying: "This means so much to me, thank you. One of the first people who taught me how to act was Denzel Washington and I just met him tonight for the first time."
Affleck ended his speech by thanking his parents for "mostly believing in me" and added: "Ben, I love you, you ain't heavy."
Many had expected the ceremony to be dominated by political speeches but references to President Donald Trump were mainly limited to jibes from Kimmel, who kicked off the ceremony by thanking the president for making the Oscars appear less racist.
He added: "I want to say thank you to President Trump. Remember last year when it seemed like the Oscars were racist? It's gone!"
The Salesman, directed by Iranian film-maker Asghar Farhadi, who is boycotting the ceremony because of Donald Trump's travel ban, has won the foreign language film Oscar.
In a statement read on his behalf, Farhadi, whose film A Separation won the same prize in 2012, said it was a "great honour" to receive the prize for a second time.
He added: "I'm sorry I'm not with you tonight. My absence is out of respect for the people of my country and those of the other six nations who have been disrespected by the inhumane law that bans entry of immigrants to the US.
"Dividing the world into the us and our enemies categories creates fear. A deceitful justification for aggression and war. These wars prevent democracy and human rights in countries which have themselves been victims of aggression."
During a break in awards, Kimmel said he was worried about President Trump because he had not yet tweeted anything about the ceremony.
He wrote on his Twitter account: "Hey @realDonaldTrump u up?" and followed it up with "@realDonaldTrump #Merylsayshi".
The president has not yet commented on the ceremony.
Tarell Alvin McCraney, who won the best adapted screenplay Oscar for Moonlight, told Radio 4 Today: "One of the things that helped was the cast and crew of La La Land being so gracious and so powerfully intimate and loving.
"They looked to us and said to us 'This is your award, you won this award'.
"There was nothing more amicable that I can imagine from them. They were in an awkward position and for them to be so gracious was staggeringly beautiful."
Despite the huge error, there was another moment at the ceremony also capturing viewers' attention, Nicole Kidman's clapping.
The way in which the Lion star applauded when an award was introduced on stage prompted much comment on Twitter.
Baratunde wrote: " Anyone see Nicole Kidman clapping? She claps like a seal. Awesome."
@steveleng wrote: " Little known fact about Nicole Kidman - she is in fact half sea lion."