Glenda Jackson, the former Hampstead and Kilburn MP, in King Lear PIC: Manuel Harlan

Glenda Jackson made a sensational speech about the “spiritual damage” wrought by Thatcherism at the late Baroness’s tribute debate in the House of Commons three years ago. Jackson’s tirade about a society riven by greed, selfishness and with “no care for the weak”, could have been ripped straight from Lear’s third act awakening.

But the ferocity she showed on that day pales compared to the menace with which she lets rip at the Old Vic.  The teeth-gnashing and writhing contortions that consume her gnarled face at every indignation – “I will do such things, what they are yet I know not, but they shall be the terrors of the earth!” – had the audience cowering in the stalls.

The 80-year-old does not wilt during this three-and-a-half hour rampage through rage and despair, although there was a collective sigh of relief when Lear collapses into sleep to balm his “broken senses”.

Jackson, who stepped down as Hampstead’s MP in 2015 after 23 years, offers an ironic cackle in the opening scene when Lear announces his intention to “shake all cares and businesses from our age” and “unburdened crawl toward death”. Glenda was not one to suffer fools, but here she is totally reliant upon one (Rhys Ifans). Ifans plays a loveably wise jester and the pair’s scenes are among the night’s best.

Simon Manyonda is a young and energetic Edmund who soliloquises while masturbating – his back to the audience – and jogs around the stage performing body-bending push-ups and yoga moves. He is less the ripened Machiavelli, more a budding meddler.

His was not the only bare bottom on show as Harry Melling (Edgar), better known as Dudley Dursley in the Harry Potter films, strips off completely and faces the audience at the front of his stage, on what was a chilly night. Jackson later uses her red coat, very similar to the one she wore for many years on the campaign trail, to cover up his modesty.

There are solid performances from Celia Imre (Goneril) and Jane Horrocks (Regan), just two of the big hitters in this stellar cast.

Director Deborah Warner – an artist who created the popular St Pancras Project installation in the Midland Hotel in the 1990s – also designed the no-frills set. It transforms briefly from a kind of blank white-walled asylum cell to a raging storm with a huge canopy of billowing bin liners and strobe-style light show. As barren as Blackheath, it helps focus the mind on what is gripping and memorable theatre.

Lear is perhaps Shakes­peare’s most overtly radical play. Shockingly for the time, a bad-tempered sovereign was reduced to a ragged, shivering outcast who cries out for others “to feel what wretches feel”. It’s all about empathy and the kind of spiritual damage Jackson was talking about in her Thatcher speech.

Like Tony Benn, it appears another MP has left Parliament to spend more time in politics.

l King Lear runs at the Old Vic Theatre, The Cut, SE1, until December 3. Box office: 0844 871 7628