An original courtroom sketch by celebrated Belfast painter Sir John Lavery, which has never been seen in public before, is to go under the hammer next month.

The piece will be auctioned in Dreweatts Modern and Contemporary Art sale at their Donnington Priory saleroom in Newbury, Berkshire, on March 13.

Lavery produced the oil sketch inside the Court of Criminal Appeal in London in 1916 during an appeal by Sir Roger Casement, who had been convicted of treason for his efforts in trying to gain German military aid for the 1916 Easter Rising in Dublin.

The work is one of a number produced by Lavery being sold by Drewatts, and has an estimate of £15,000 to £25,000.

It was used as a preparatory study for Lavery’s famous later painting The Hearing of the Appeal of Sir Roger Casement.

The artist created the study with Casement, who was hanged in 1916, looking straight out towards the jury box.

Art historian Kenneth McConkey said: “For those two days Lavery, accompanied by his wife Hazel, sat in the witness box recording the scene in the present sketch.

“During the painful excursion into a legal precedent deriving from a 14th century statute on treason, Lavery’s concentration on the scene before him was intense.

“Although he made efforts to conceal his industry, the production of the present 10in x 14in canvas board in an awkward space was detected by the press, as well as by the prisoner in the dock facing him.”

Also on sale will be two portraits from the family of the presiding judge in the appeal, Sir Charles Darling, and two other works – one a view from Lavery’s house at Tangier overlooking the Strait of Gibraltar, and an oil sketch for his celebrated portrait of Mrs Roger Plowden and Humphrey from 1897.

Co-head of Dreweatts Modern and Contemporary Art Jennie Fisher said: “We are thrilled to be able to offer a cross-section of works demonstrating the breadth and range of Lavery’s work, from an evocative example of his plein air landscape painting, to society portraits, through to a rare and important study for one of his most important historical commissions.

“The preparatory study of The Hearing of the Appeal of Sir Roger Casement is undoubtedly the highlight of the group and significant not only for its superb draughtsmanship, but for its portrayal of a hugely important moment in Anglo-Irish history.”

Lavery was born in a small spirit-grocer’s shop in North Queen Street, Belfast, in 1856 and was orphaned at the age of three.

After training as an artist in France he found fame after he was commissioned to paint the state visit of Queen Victoria to the International Exhibition in Paris, and later painted George V and his family. He taught Winston Churchill to paint.