Roy Precious - Antiques & Fine Art has dated this piece to the 1800s.
Westmacott (portrait also on this website).

But there were four brothers in the army...Daniel, John, Charles and Thomas ...a lieutenant, a captain and two majors; however this portrait has decended directly through the family and is known as Daniel.

The handwritten label verso says the sitter was a captain in the 24th Dragoons, but the dragoons wore blue uniforms and this officer wears a red tunic with the yellow facings of a Scottish regiment. Of course, the officer may have changed regiments at some time but clearly more research is needed to identify the sitter beyond doubt.

In August 1822 two of the Stewart brothers died in India, both in the Honourable East India Company, Bombay; Major John Stewart died of a liver complaint aged 43, and Captain John Guise Stewart died of a wound in the head aged 38,"both brothers of Mrs. H. Westmacott of Cadogan-place". Reported in 'The Gentleman's Magazine' Volume 92, "Obituary of Remarkable Persons" and "Blackwood's Magazine" Volume 11.

The parents of the sitter were Thomas Stewart, Town Clerk of Montrose and Elizabeth Guise, born in Gibraltar in 1755.
The National Portrait Gallery has Thomas Phillip's sitter's book in which this portrait and that of his sister Elizabeth Stewart are included.
THOMAS PHILLIPS R.A. (1770-1845) was a leading English portrait painter. He painted many of the important and wealthy of the day including scientists, artists, writers, poets and explorers.
He was elected ARA 1804, RA in 1808, becoming Royal Academy Professor of Painting in 1825.
Phillips was a contemporary of Lawrence and offered a more subdued style based on his study of Old Masters. "His best works are full of character and show him to be in the top rank of portrait painters" (The Dictionary of Portrait Painters in Britain up to 1920.)
SIZE: 40.5 x 35.75 inches inc. frame
30 x 25 inches canvas size.
PROVENANCE: By family descent.
Deceased Estate, Dorset.
VERSO: handwritten label identifying the sitter.
Roy Precious - Antiques & Fine Art has dated this piece to the 1800s.
Roy and nadine precious: specialists in portraiture from the 16th to the 20th century. Established in 1972.
You can view the stock and full dealer information on the Roy Precious - Antiques & Fine Art Profile Page