The tribunal found the man, a resident of the city of Barnaul in the Altai region of Siberia, guilty of "attempted assault" and "attempted high treason" and sentenced him to 20 years in prison, the RIA Novosti state news agency quoted the FSB as saying in a statement.

"This Russian citizen, born in 1981, entered into contact with Ukrainian special services via social media" and expressed his "desire to support Ukrainian armed forces", the statement said.

"As an employee of a canteen, he tried to poison the soldiers who were part of a mobilised contingent in the Altai", filming the process for the Ukrainian special forces, it said.

Analyses showed that the substance that he put in the food could have caused acute poisoning and could have been fatal, it said.

The case is the latest in a long line of Russians handed heavy prison sentences for allegedly supporting Ukraine after Moscow sent troops into its neighbour in February 2022.

Since then, thousands of people in Russia have been sanctioned for their opposition to the conflict.