Sir Ian McKellen’s show must go on.

A new production of “Hamlet,” starring the 81-year-old actor in the title role, will begin rehearsals Monday in Windsor, England, The Post has learned.

It will be the first major play in the English-speaking world to get back to work in the wake of the coronavirus pandemic.

Put on by powerful producer Bill Kenwright, the age-defying Shakespeare show will begin performances at the Theatre Royal Windsor — steps from where Queen Elizabeth II is quarantined — on a date to be announced.

“I feel lucky to be working again, thanks to Bill Kenwright’s inspiring optimism and [director] Sean Mathias’ invitation to re-examine ‘Hamlet,’ 50 years on from my first go,” McKellen said in a statement. “So now we will meet again. Don’t know when but do know where – Theatre Royal Windsor!”

The rehearsals, Kenwright said, will strictly adhere to government guidelines on safety and social distancing.

Mathias, who directed McKellen and Patrick Stewart in 2013’s “Waiting for Godot” and “No Man’s Land” on Broadway, said, “The disappointment at being halted by COVID-19 has now been replaced with encouraging signs that we can at least start to work on these beautiful plays with an exceptional company.”

McKellen will be joined by Ben Allen, Emmanuella Cole, Alis Wyn Davies, Oli Higginson, Nick Howard-Brown, Jonathan Hyde, Asif Khan, Missy Malek and Jenny Seagrove.

After “Hamlet,” McKellen will take on the role of Firs in Anton Chekov’s “The Cherry Orchard,” also at the Theatre Royal Windsor.

Broadway, meanwhile, has been shuttered since March 12, due to the pandemic, and is currently slated to be dark till Labor Day.