Beresford was honoured and decorated by the governments of Great Britain, Portugal and Spain.
He served as commander in chief of the Portuguese army for 11 years. The Iron Duke once referred to him as “the ablest man I have yet seen in the army”.
Modestly estimated at £100-150, the portrait of an elderly Beresford was contested by two phone bidders to £9500 where it was knocked down at Hansons (20% buyer’s premium) in Staffordshire on December 12.
An old label to the verso of the 2ft 9in x 2ft 1in (85 x 65cm) oil on canvas attributed the portrait to the Birmingham-born painter Reuben Sayers (1815-88).
Mezzotints published in 1850 after a Sayers portrait very similar to the one sold at Hansons exist in the collections of the National Portrait Gallery, the Royal Collection and the British Embassy in Warsaw.
Elsewhere in the sale, an oil portrait depicting Christiane Eberhardine (1671-1727) of Brandenburg-Bayreuth and Electress of Saxony, in a jewelled dress and urmine trimmed cloak, sold for £2100 (£400-600 estimate).
The 2ft 8in x 2ft 1in (82 x 64cm) oil on canvas was discovered on a routine home visit in Gloucestershire and is thought to be a 19th century version of an 18th century portrait by Louis de Silvestre (1675-1760).