A Norwegian appeals court has sentenced one of the country’s best-known artists, Odd Nerdrum, to two and a half years in jail for tax evasion.

The Borgarting Court of Appeal found the artist guilty of selling paintings abroad for 14 million kroner ($2.3 million) in the period 1998-2002 without declaring any of the income he received to the Norwegian tax authorities.

Last August, Oslo District Court handed Nerdrum a two-year sentence, but the renowned figurative painter hoped to be cleared of the charges against him on appeal.

Instead, the appeals court on Wednesday lengthened his sentence in line with the wishes of the prosecution.

Nerdrum’s lawyer, Pål Berg, described the verdict as “completely insane”.

The case centred on a series of paintings Nerdrum sold through a gallery in the United States. The artist said collectors had complained that the paint he used on the pictures ran when exposed to heat, forcing him to give the buyers a five-year money-back guarantee in case there was any problem with their purchases.

The 68-year-old artist said he reached an agreement with the US gallery to place his earnings, cheques worth $900,000, in an Austrian safe deposit box. This meant the money would be available to return to any buyers unhappy with the quality of the paintings.

“I made an oral agreement about this. The money was to remain in the bank safe for five years before it would be sent to me as income for taxation,” Nerdrum told the court.

The prosecution did not believe this version of events, contending instead that Nerdrum stowed the money in Austria specifically to avoid paying any tax.