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The actress took home the Best Supporting Actress award for her role as Margaret Thatcher in Netflix's The Crown.
But, to many viewers' surprise, she spoke with an American accent as she accepted the award.
Gillian has lived between the US and the UK for most of her life. She was born in the US, moved to London when she was a child and eventually moved back to America aged 11.
She has played a range of British characters including her recent portrayal of Margaret Thatcher in the royal drama and her role of Stella Gibson in The Fall.
Reacting to her American accent, fans from the UK flocked to Twitter to express their confusion.
One person on Twitter said: "I'm always so surprised hearing Gillian Anderson's American accent."
Another person added: "I watched the full run of The X-Files, but after The Crown, Sex Education, I am at the point where I am legit surprised to hear Gillian Anderson speak in an American accent."
"It's always interesting to see which accent Gillian Anderson will pull out," a third wrote.
Having lives in the US and on British soil, Gillian often switches between an American and British accent.
In the past she has addressed her unusual accent in an interview with The Telegraph.
"Even on the phone my accent will change," she said. "Part of me wishes I could control it, but I can't. I just slip into one or the other. When I moved to the States I tried hard to cling on to my British accent because it made me different."
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