structural:summarization, stylistic:tone
Illness

By this time, George's health was deteriorating. He had a mental illness characterised by acute mania. Until the mid-20th century, the King's illness was generally considered to be psychological. In 1966, a study by Ida Macalpine and Richard Hunter suggested that the illness was physiological, caused by the liver disorder porphyria. Although meeting with some contemporary opposition, the view gained widespread scholarly acceptance. A study of samples of George's hair published in 2005 revealed high levels of arsenic, a cause of metabolic blood disorders and thus a possible trigger for porphyria. The source of the arsenic is not known, but it could have been a component of medicines or cosmetics. The theory was also established in the public mind through influential dramatisations, such as Alan Bennett 's play The Madness of George III, and in Nicholas Hytner 's subsequent film. From 2010 this view has been increasingly challenged, and Macalpine and Hunter's study criticised. Recent scholarship discounts the porphyria theory and contends that George's illness was psychiatric, most probably bipolar disorder.

George may have had a brief episode of disease in 1765, and a longer episode began in the summer of 1788. At the end of the parliamentary session, he went to Cheltenham Spa to recuperate and in August visited the Bishop of Worcester at Hartlebury Castle and Viscount Mount Edgcumbe at Cotehele, Cornwall, with the Queen, and their daughters the Princess Royal and Princesses Augusta and Elizabeth. It was the furthest he had ever been from London, but his condition worsened. In November of that year, he became seriously deranged, sometimes speaking for many hours without pause, causing him to foam at the mouth and his voice to become hoarse. George would frequently repeat himself and write sentences with over 400 words at a time, and his vocabulary became "more complex, creative and colourful", possible symptoms of bipolar disorder. His doctors were largely at a loss to explain his illness, and spurious stories about his condition spread, such as the claim that he had shaken hands with a tree in the mistaken belief that it was the King of Prussia. Treatment for mental illness was primitive by modern standards; George's doctors, who included Francis Willis, treated the King by forcibly restraining him until he was calm, or applying caustic poultices to draw out "evil humours".

In the reconvened Parliament, Fox and Pitt wrangled over the terms of a regency during the King's incapacity. While both agreed that it would be most reasonable for the Prince of Wales to act as regent, Fox suggested, to Pitt's consternation, that it was the Prince's absolute right to act on his ill father's behalf with full powers. Pitt, fearing he would be removed from office if the Prince of Wales were empowered, argued that it was for Parliament to nominate a regent, and wanted to restrict the regent's authority. In February 1789, the Regency Bill, authorising the Prince of Wales to act as regent, was introduced and passed in the House of Commons, but before the House of Lords could pass the bill, George recovered.