The Madness Of George III, West Yorkshire Playhouse, Leeds

What can Michael Pennington not do? In recent months, he has toured Britain as Ibsen's John Gabriel Borkman, turned in a sterling performance of Dr Dorn in Peter Stein's Seagull at the Edinburgh Festival, directed A Midsummer Night's Dream for Regent's Park Open Air Theatre and brought out another book. Now he has assumed the throne in a large-scale production of Alan Bennett's drama The Madness of George III, brought about through a pooling of resources by West Yorkshire Playhouse and Birmingham Rep.

The mention of the king who lost his mind as well as the American colonies inevitably conjures up the late Nigel Hawthorne's performance on stage and screen, but Pennington presents the troubled monarch with absolute authority. His portrayal of the king's deterioration into, first, confusion, then babbling incoherence and, finally, delirium is frighteningly realistic. While never quite foaming at the mouth, Pennington assumes, brilliantly, all the received characteristics of someone teetering on the brink of madness, slipping between lucidity and insanity. The verbal tics - his habit of filling any empty sentence with "yes-yes!" or "what-what!" is maddening enough when he is sane - descend into torrents of verbal diarrhoea as his mind becomes unhinged.

The genial, inquisitive dimension of the king's character is played with tremendous warmth: his affection for his longtime queen, Charlotte of Mecklinburg-Strelitz (Alison Fiske), his delight in teasing his sombre, fiercely loyal Prime Minister, Pitt the Younger (Paul Raffield), his dismissive attitude toward his greedy eldest son, George, Prince Regent (foppishly presented by Stewart Wright), and his disdain for the Whigs preparing for government. Most touching is the sometimes wry, sometimes gleeful smile glinting behind the barbed comments and ferocious attacks on those around him, his most blistering gibes reserved for the hopeless doctors charged with curing him.

An enormous flight of red stairs dominates the thrust stage, a golden sow and two piglets reminding us that "Farmer George" had cultivated a common touch with his land and people. A huge net curtain and staircase landing shift us from court to Parliament, from the king's bedroom to the chilly Kew palace, from the Regent's frivolous partying to the meetings of machinating ministers. Francis O'Connor's uncluttered design is matched by Rachel Kavanaugh's telling direction, focusing on the king's struggle to hold on to both his wits and his throne. There isn't a weak performance among the 20-strong cast, from the humblest footmen to the substantial parts of the royal retinue, particularly Andy Hockley and Paul Kemp as Fox and Sheridan, Timothy Knightley as the toadying doctor Sir George Baker, Julia St John as a gracious Lady Pembroke and Alastair Cording as a canny Dundas.

With bursts of music by Handel, the king's favourite composer, separating the scenes, and the occasional image cleverly frozen into the next, the action moves swiftly. Kavanaugh brings out the wit and the pathos. But never, as Birmingham Rep's website mistakenly claims, is it a hilarious comedy. Bennett's compelling chronicle of an ailing man destined to a 10-year living death - blind, deaf and mad - and a historic struggle for power in government is far too disturbing for that. It has so many resonances that it could have been written this year - from the instruction to the therapist Dr Willis (a muscular performance by Ken Drury) to "rewrite the bulletin... tone it down", to the quip that "office makes Tories of us all".

To 18 October (0113-213 7700); at Birmingham Rep (0121-236 4455) 22 Oct-15 Nov