She choked back tears as she was presented with the award for Album of the Year for "21" at the end of the night from George Michael, telling the audience, "Thank you very, very much. Nothing makes me prouder than coming home with six Grammys and then coming to the Brits and winning Album of the Year. I'm so, so proud to be British and to be flying our flag."
However, her big moment was soured when host James Corden cut short her acceptance speech to make way for closing act Blur, prompting the crowd to boo, while an angry Adele made the obscene gesture (video below) to show her feelings over the move.
As she was rushed off stage, she said, "Can I just say, then, goodbye and I'll see you next time round."
After the incident, the superstar defended her actions, saying, "I flung the middle finger. That was for the suits (organisers) at the Brit Awards, not my fans. I'm sorry if I offended anyone but the suits offended me. I was about to thank the British public for all their support but they cut me off. So thank you very much for all your support. It's amazing, so thank you very much."
Rising star Ed Sheeran was also celebrating a double delight as he was named British Male Solo Artist and British Breakthrough Act, and Coldplay won the British Group prize - the third time they've scooped the honor, taking their career total of Brit Awards to seven.
Bruno Mars was named International Male Solo Artist, and Rihanna took the award for International Female Solo Artist for the second year running, ahead of fellow stars including Lady Gaga and Beyonce.
The "Umbrella" hitmaker told the audience, "To all my fans who continue to support me no matter what, I love you - this is for you. My fans always remind me to be myself and I'll never forget you for that."
Coldplay opened the event at the O2 Arena in London with a performance of new single "Charlie Brown", followed by Florence and the Machine with "No Light, No Light".
Adele performed her hit "Rolling in the Deep"; Rihanna wowed the crowd with a raunchy rendition of "We Found Love", during which she emerged from a paint-spattered glass box wearing white hotpants and Timberland boots; and Noel Gallagher performed his single "AKA... What a Life!".
Guests were also treated to poignant video tributes to Amy Winehouse, who died in July last year, and Whitney Houston, who passed away earlier this month.
Blur, who set a record in 1995 for the most Brit wins at one ceremony by scooping a total of four, received the Outstanding Contribution to Music honor before performing several of their hits, including "Parklife" and "Song 2".
Receiving the award from movie hardman Ray Winstone, frontman Damon Albarn said, "Last time we were here was 17 years ago, and what happened that night seemed to have a really profound effect on our lives, so it's very nice to come back and say thank you very much for this honor."
The full list of winners is as follows: British Male Solo Artist - Ed Sheeran British Female Solo Artist - Adele British Breakthrough Act - Ed Sheeran British Group - Coldplay British Single - One Direction ("What Makes You Beautiful") British Album of the Year - Adele ("21") International Male Solo Artist - Bruno Mars International Female Solo Artist - Rihanna International Group - Foo Fighters International Breakthrough Act - Lana Del Rey Outstanding Contribution to Music - Blur Critics Choice - Emeli Sande British Producer - Ethan Johns