UN experts have called for Julian Assange to be released from prison and criticised the British government for breaching his human rights.

The WikiLeaks publisher was jailed for 50 weeks on Wednesday for breaking bail conditions imposed seven years earlier by seeking asylum in the Ecuadorian embassy in London.

The UN working group on arbitrary detention (WGAD) said it was deeply concerned by the “disproportionate sentence” imposed on Assange for violating the terms of his bail, which it described as a “minor violation”.

The group has twice previously called for Assange to be freed, after it judged his confinement to the Ecuadorian embassy by the threat of arrest should he leave amounted to arbitrary detention.

“The working group regrets that the government has not complied with its opinion and has now furthered the arbitrary deprivation of liberty of Mr Assange,” it said in a statement on Friday.

“It is worth recalling that the detention and the subsequent bail of Mr Assange in the UK were connected to preliminary investigations initiated in 2010 by a prosecutor in Sweden. It is equally worth noting that that prosecutor did not press any charges against Mr Assange and that in 2017, after interviewing him in the Ecuadorian embassy in London, she discontinued investigations and brought an end to the case.

“The working group is further concerned that Mr Assange has been detained since 11 April 2019 in Belmarsh prison, a high-security prison, as if he were convicted for a serious criminal offence. This treatment appears to contravene the principles of necessity and proportionality envisaged by the human rights standards.

“The WGAD reiterates its recommendation to the government of the United Kingdom, as expressed in its opinion 54/2015, and its 21 December 2018 statement, that the right of Mr Assange to personal liberty should be restored.”

A government spokesperson said: “The UK has a close working relationship with UN bodies and is committed to upholding the rule of law. Sentencing is a matter for our independent judges, who take into account the full facts of each case, and the law provides those convicted with a right of appeal.”

Assange appeared in court on Thursday via video link from Belmarsh as he began a legal fight against extradition to the US, where he is wanted on charges relating to the publication of secret US files leaked by Chelsea Manning, a US intelligence analyst who was subsequently jailed.

They included approximately 90,000 reports about the war in Afghanistan, 400,000 Iraq war reports and 800,000 Guantánamo Bay detainee assessments, as well as US diplomatic cables.

Assange declined to consent to his extradition, saying: “I do not wish to surrender myself for extradition for doing journalism that has won many, many awards and protected many, many people.”