The heist of a famed portrait of a scowling Winston Churchill has gripped Canada's capital since it was discovered the photograph hanging at an Ottawa hotel for decades had been swapped for a fake.

Police were called in after staff at the Fairmont Chateau Laurier hotel in Ottawa last Friday noticed the picture of the late British prime minister was askew and didn't match those of other portraits gifted by the late Armenian-born Canadian photographer Yousuf Karsh.

The "Roaring Lion" portrait was taken by Karsh after the wartime leader addressed the Canadian parliament in 1941, becoming a symbol of British defiance in the Second World War.

As speculation swirls over the heist, former hotel guests have shared their snaps of the portrait over the years, helping to narrow down the date when it could have gone missing from December 25, 2021 to January 6, 2022.

"Somebody probably wanted that picture either for their private collection or to sell it. I don't know," Genevieve Dumas, general manager at the Fairmont Chateau Laurier, said.

The portrait is estimated to be worth US$100,000, but Dumas said it is priceless.

"It means a lot to us. It's part of Karsh history, the hotel's history as well as Canadian and British wartime history," she said.

"We're deeply saddened by this brazen theft," she added. "We just hope to get it back."

Karsh and his wife, after fleeing the Armenian genocide and settling in Canada, lived at the hotel for 18 years. He also had a studio there until 1992.

His other portrait subjects included the Reverend Martin Luther King, Jr, Albert Einstein, Ernest Hemingway and Queen Elizabeth II.

According to historical accounts, Karsh plucked a cigar from Churchill's mouth just before taking his portrait, which made the British premier grimace.

The image is arguably the most iconic of Churchill and widely circulated, even appearing on the British five pound note.

Dumas described how maintenance staff had been the first to notice something wasn't right with the portrait that hung in a reading room adjacent to the main lobby.

Police are now reviewing security footage, but because the theft occurred when Covid-19 restrictions were in place, Dumas said the thief likely wore a mask. (AFP)