LONDON - A dispatch penned by British explorer Captain James Cook on his return from his first epic voyage to Australia more than two centuries ago has been found in the back of a picture frame in an English country house.

Bonhams Auction House, which is selling the contents of Brancaster Hall in Norfolk, eastern England, announced the find on Tuesday. It said it expected the dispatch to sell for between 15,000 pounds and 20,000 pounds ($23,000-$31,000).

The auction is to take place on September 9 at Brancaster Hall, Reuters reported.

James Glennie of Bonhams found the letter to the British admiralty as he was listing the contents of the house, which has been owned by a single family for 300 years and never been opened to the public.

"I noticed a small, framed document hanging in a dark corner of the library and was intrigued to find it was Cook's bill to the treasury for supplies," he said in a statement.

"However, I was astonished to discover on the reverse of the frame the letter signed by Captain Cook," he continued.

The previously unrecorded dispatch, signed in Cook's distinctive flowing hand, is believed by Bonhams to have been written as his ship the Endeavour neared the coast at Kent -- Cook's first sight of England for three years.

By that time malaria and dysentry had reduced the crew by a third. "Parcel containing 17 monthly muster books and 29 dead tickets," Cook recorded in his copperplate handwriting. The tickets were important as he would have had to return them to the admiralty when he reported the fate of his crew.

The explorer originally set off in 1768 on a scientific expedition to Tahiti in the South Seas to observe the transit of venus across the sun.

Later, however, he received new orders from the admiralty to find out if another southern continent existed between New Zealand and Antarctica.

After a hostile reception from the Maoris in New Zealand and being blown off course trying to get to Tasmania, the Endeavour finally sighted land at the southeast coast of Australia.

The crew spent months mapping some 2,000 miles (3,200 km) of coastline and recording new species of plants and animals before making their epic return voyage.

It is not known how the documents came to be in Brancaster Hall, but the family who own it have connections with Whitby in Yorkshire, northeast England, where Cook learned his trade as a seaman.