It will be offered in its sale on March 7-8 with an estimate of £80,000-120,000.
The 22in x 2ft 6in (56 x 76cm) oil on canvas depicts an Italian mastiff in a mountainous landscape. While dogs feature in other Guercino paintings, the only other known canine portrait is Portrait of the Aldrovandi Dog which sold at auction in the 1970s and is now in the Norton Simon Museum in Pasadena, California.
This latest painting was bought by the vendor’s great-grandfather in Rome in 1850 and has resided in a Suffolk country house for the past 167 years.
While previously unknown to scholars, the attribution to il Guercino has been endorsed by Dr Nicholas Turner, an authority on the artist’s work, and by John Somerville, senior curator of the Lobkowicz Collection, Prague.