UN human rights experts say Russia violated international law by imprisoning Wall Street Journal reporter Evan Gershkovich and should release him “immediately”.

The Working Group on Arbitrary Detention, made up of independent experts convened by the UN’s top human rights body, said there was a “striking lack of any factual or legal substantiation” for spying charges levelled against Mr Gershkovich, 32.

The five-member group said Mr Gershkovich’s US nationality has been a factor in his detention, and as a result the case against him was “discriminatory”.

“The Working Group finds that Mr Gershkovich’s deprivation of liberty constitutes a violation of international law on the grounds of discrimination based on his nationality,” the group said in a decision that was taken in March but made public only on Tuesday.

Matthew Gillett, the working group’s chairman, said its opinion was grounded in the International Covenant on Civil and Political Rights, which was adopted in 1966 and nearly all UN member countries have ratified.

“The covenant is something that Russia has freely signed up to and accepted the obligations under, and therefore as a matter of international law, it is obliged to implement the provisions of the covenant,” he said in an interview.

Mr Gillett said Russia should provide Mr Gershkovich with “proper reparations” for holding him for over a year in detention without a legitimate basis.