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The 23-year-old was midway through an acceptance speech at the packed O2 Arena in her native London after picking up the big prize - best British album for her chart-conquering "21".
But because the show was running late, and being broadcast live on ITV, presenter James Corden stepped in to cut Adele short, prompting the "one finger salute".
The Sun newspaper, Britain's best-selling daily, had a picture of the gesture on its front page above the headline "Someone dislikes you", a reference to her hit "Someone Like You".
The rival Mirror featured a similar image accompanied by the words "Adele: Go to hell!"
Speaking backstage after the two-hour show, Adele said she was upset by the interruption.
"I flipped the finger but it wasn't to my fans," she told the Sun. "I'm sorry if I offended anyone, it was the suits that offended me."
The incident comes weeks after U.S. television network NBC and the NFL apologised for an offensive finger gesture made by British rapper M.I.A. during singer Madonna's televised show at the Super Bowl.
Madonna slammed M.I.A.'s behaviour as juvenile, negative and out of place.
Both ITV and the BRIT Awards issued apologies following the show, which ended on a sour note after what had been another triumphant night for Adele.
Fresh from her record-equalling six-Grammy haul in the United States earlier this month and return to live performing after surgery on her vocal cords late last year, Adele bagged two BRITs -- best British female and best British album.
"Nothing makes me prouder than coming home with six Grammys, then coming to the BRITs and winning album of the year," she said to loud cheers after winning the album prize, considered the most important.
"I'm so proud to be British and to be flying the flag and I'm so proud to be in the room with all of you." At that point she was cut short, prompting boos in the crowd.
There was some good news for TV bosses -- audience ratings showed a peak viewing figure of 7.4 million, the highest in seven years. The average audience was 6.2 million.
Yet some critics felt the night was more about "suits" - corporate executives focussing on record sales and viewing figures - than music.
Performers included Adele with "Rolling In the Deep", Ed Sheeran with "Lego House" and Coldplay with "Charlie Brown".
Andy Gill of the Independent newspaper called the awards "perhaps the dreariest two hours that TV viewers have sat through in decades."
He wrote in his review that every award winner was predictable and many performers, including Coldplay, Florence + The Machine and Noel Gallagher, were dull.
Rihanna livened up proceedings when she emerged from a transparent box smeared in paint, while Adele and singer-songwriter Ed Sheeran put others to shame, Gill said.
Sheeran, a 21-year-old who sang accompanied by an acoustic guitar, was the other big winner on the night, scooping the British breakthrough and British male solo categories.
Both he and Adele have been disparagingly referred to as the "New Boring" in British pop, although they are selling plenty of records which will cheer an industry in decline for the best part of a decade.
In its 52nd week of release ending Feb. 19, Adele's 21 album had its largest U.S. sales week with 730,000 albums sold, more than the previous high of 399,000 copies in the week ending Dec. 25, 2011, according to industry benchmark Nielsen SoundScan.
The record has sold more than 15 million copies worldwide.