A handwritten draft letter of abdication penned by King George III is to go on display for the first time as part of a huge cache of documents from the reign of the last British monarch to rule over America, the royal household said.

The draft letter of abdication written by King George III.

The letter, covered in blotches, scratchings and corrections, was penned during a period of political crisis in March 1783 at the end of the American War of Independence.

"I am therefore resolved to resign my Crown and all the Dominions appertaining to it to the Prince of Wales, my eldest son and lawful successor," said the letter, which was never sent.

It is among a collection of 350,000 Georgian papers from the royal archives which have been digitally scanned and will be placed on an online portal for historians and the public to study from today.

The documents include intimate letters between George and his wife Charlotte, household bills and a huge number of letters between the king and the government with detailed notes about the war in America.

There is also an instruction manual from George's father about kingship.

George, Britain's longest-reigning king who was on the throne from 1760 until 1820,  in the latter part of his life which was documented in the award-winning 1994 film The Madness Of King George.

Current heir-to-the-throne Prince Charles has said George was unfairly remembered by history as mad and the king who lost America, when he was in fact one of Britain's most dutiful, cultured and misunderstood rulers.