A MAJOR portrait of Archibald Campbell, 3rd Duke of Argyll, the founder of the Royal Bank of ­Scotland, and one of the 31 Scottish Commissioners who negotiated the 1707 Act of Union with England, is expected to fetch up to £300,000 at auction next week.

The 1758 painting is by Allan Ramsay, one of the greatest and most influential portrait painters of the 18th century.

Campbell, who lived from 1682 to 1761, was a nobleman, politician, lawyer, businessman and soldier. The painting is to be sold at Sotheby's in London on July 9.