A portrait by the Sudbury-born artist Thomas Gainsborough, estimated at £120,000 to £180,000, is up for sale.

The painting, widely believed to be of General James Wolfe, who died leading his troops at the Battle of Quebec, is to be offered at Bonhams Old Master Paintings Sale in London December 3.

A spokesman for Bonhams said: “The portrait poses something of a mystery. It was almost certainly commissioned by the family of Katherine Lowther who became Wolfe’s fiancée in 1758 during his preparations for the military expedition to Quebec.

“Katherine was the sister of James Lowther, later Earl of Lonsdale, and she and Wolfe had met casually before. They renewed their acquaintance while he was on leave in Bath and by the time he sailed for Canada in 1759 they were engaged to be married.”

The spokesman said all correspondence between the pair has now been lost so it is impossible to know the depth of their feelings for each other. Some experts have suggested that Wolfe may have been principally concerned with placating his ailing parents who were keen to see their son settle down and marry advantageously.

They had deeply disapproved when, in 1747, he had fallen in love with Elizabeth Lawson, a niece of his brigade commander in Flanders, Sir John Mordaunt.

“This led to a family rift and a prolonged bout of dissipation on Wolfe’s part as he attempted to drown his sorrows”, the spokesman added. “Later, while visiting his uncle, Major Walter Wolfe, in Dublin, James met an unnamed widow of an officer and the close nature of their relationship is evident from the fact that she would later be lampooned by George Townshend as Wolfe’s ‘Irish Venus’.”

Wolfe’s role in the capture of the city, the subsequent capture of Montreal and the effective end of the Seven Years War between Britain and France made him a national hero.