His Royal Highness The Prince Regent
In 1788, King George III's mental health greatly deteriorated and it began his ultimate decline and Parliament started to discuss the idea of a Regent and who would be a best choice to be Regent. Before The Regency Bill could be passed King George III recovered.
In 1795, George married his cousin Princess Caroline of Brunswick. In 1796, their only child, Princess Charlotte was born. Shortly after her birth they formally separated and George continued his on and off relationship with Maria Fitzherbert.
In 1810, King George's health declined again and this time the billed passed to become the Regency Act of 1811. George became the Prince Regent on February 5, 1811. He 'ruled' in his father's place until King George died in 1820.
The Prince Regent became King George the IV on July 19, 1821.
For more information on Prinny you can read his Wikipedia article - George IV of the United Kingdom
For further reading there is a list at the bottom of the page or you can jump to it right here - References and further reading