LONDON — It’s one of the most demanding roles in theater, but Glenda Jackson, the two-time Oscar-winning actress, will open a new production of “King Lear” on Friday having not stepped onstage for 25 years. In the interim, she worked to keep a realm together as a member of the British Parliament; her first act in her return to the theater will be to play a monarch who breaks one up.

Many major actors take a run at the role in Shakespeare’s tragedy, a part so daunting that it’s nicknamed Mount Lear. It would seem especially so for Ms. Jackson, whose last performance came in 1991, in Eugene O’Neill’s “Mourning Becomes Electra” at the Glasgow Citizens Theater. Her return, after such a long hiatus, is highly unusual and, at 80, she is older than all but one of Britain’s last 10 Lears in major productions.

What’s more, she’s playing him at the Old Vic — a 198-year-old, 1,000-seat theater with an imposing history. Laurence Olivier was Othello there; Judi Dench was Juliet. “She’s part of that tradition now,” its artistic director, Matthew Warchus, said.

Before starting his job, Mr. Warchus invited Ms. Jackson for a meeting. He had tried to tempt her back into acting while she was still in Parliament, to no avail, and he arrived with “very, very low expectations.”