British Prime Minister Boris Johnson has described the court ruling in the case of Russian opposition activist Alexei Navalny as "cowardice" and called for Navalny's immediate release.

"Alexey Navalny's decision to return to Russia after being poisoned was a truly brave and selfless act. In contrast, today's ruling was pure cowardice and fails to meet the most basic standards of justice. Alexey Navalny must be released immediately," Johnson tweeted.

The call for Navalny's release was earlier made by British Foreign Secretary Dominic Raab.

"The UK calls for the immediate and unconditional release of Navalny and all of the peaceful protesters and journalists arrested over the last two weeks." Raab said.

The court decision "shows Russia is failing to meet the most basic commitments expected of any responsible member of the international community," he said.

A court in Moscow replaced Navalny's suspended sentence with 3.5 years in a penitentiary on Tuesday.