But for many younger fans in the 21st century, she was best known as Professor McGonagall in all seven Harry Potter movies, and the Dowager Countess in the hit TV series and movie spin-offs of Downton Abbey, a role that seemed tailor-made for an actress known for purse-lipped asides and malicious cracks.

Smith’s waspish turn in the smash-hit historical series Downton Abbey was the best reason to watch it, and it earned her multiple awards - although it did little for her desire for a private life.

“I led a perfectly normal life until Downton Abbey,” she said at the BFI Radio Times festival in 2017.

“I’m not kidding. I’d go to theatres, I’d go to galleries, things like that, on my own. And now I can’t and that’s awful.”

Smith was known for being demanding on herself and others. Theatre director Peter Hall, who worked closely with her for many years, said: “”She nags herself into perfection.”

She had a tempestuous eight-year marriage to actor Robert Stephens, which ended while they were playing newly entangled divorcees in Noel Coward’s Private Lives. They had two sons - actors Toby Stephens and Chris Larkin.

Smith then married her teenage sweetheart, writer Beverley Cross, a rock of imperturbability for her until his death in 1998.

In 1990 Smith was knighted by Queen Elizabeth and became a Dame.

British Prime Minister Keir Starmer said Smith “introduced us to new worlds with the countless stories she acted over her long career”.

“She was beloved by so many for her great talent, becoming a true national treasure whose work will be cherished for generations to come,” he said.

AP and Reuters