A letter written by Northern explorer Captain Cook informing the Admiralty of his safe return from his first epic voyage has come to light after more than 200 years.

The missive, written after his mammoth sail to Australia on the Endeavour, was found stuck to the back of a picture frame, where it is believed to have been hidden for more than two centuries.

The unique manuscript, which could be worth as much as £20,000, relates to what many historians consider to be the most significant voyage of the 18th century.

It was found last week by a valuer from Bonhams auctioneers during preparations for an unrelated sale.

The letter was attached to the back of a framed list of supplies - Cook's bill to the Treasury for supplies purchased for the ships Discovery and Resolution during his second voyage in 1776.

The lost letter is thought to have been written by Middlesbrough-born Cook as he approached England somewhere off the Kent coast in June 1771 and was almost certainly his first communication with the Admiralty on returning to British waters.

It was the end of a three-year voyage during which he charted the coasts of New Zealand, the east coast of Australia, discovered Botany Bay and travelled to Tahiti to observe the transit of the planet Venus across the sun - the principal aim of the expedition.

David Park, head of the books and manuscripts department at Bonhams in London, where the items will be sold in a specialist manuscript sale on December 17, said: "There must have been a great deal of satisfaction for Cook in writing such a letter to the Admiralty saying 'I've made it and I'm nearly home' - it is almost the equivalent of the Star Trek crew returning to Earth."

The letter underlines that the expedition was far from plain sailing. On the return voyage, malarial fever and dysentery broke out, depleting the crew by a third. By the time the Endeavour dropped anchor off the Kent coast only Cook and 55 crew survived.

It is not certain how the letter or the framed list, which has been separately estimated at £3,000 - 4,000, came to be at Brancaster Hall.