Colin Firth, is that you?

The 55-year-old actor stepped out looking noticeably thinner at a London Fashion Week event on Sept. 21, leading many fans to worry about his gaunt appearance.

colin firth weight loss

Colin in December 1998 (left) and September 2015 (right). (Photo Credits: Getty Images)

Though the reason for Colin's weight loss is unclear, the actor has been known to drastically alter his physique in the past for film roles including 2013's The Railway Man — in which he played a prisoner of war — and 2014’s Kingsman: The Secret Service.

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In September 2013, his wife of nearly 20 years, producer and director Livia Firth, took to Twitter to address her husband's thinner frame.

"Re Colin’s weight speculations, he has been asked to train 4 Matthew Vaughn movie!" she wrote. "Feeling fit but can’t wait to expand again after Xmas!"

According to the Daily Mail, the Oscar winner's reps declined to comment on Colin's weight loss or health at this time.