Julian Assange is in arbitrary detention, UN panel finds …

Julian Assange argued he was illegally confined to the embassy because he risked arrest if he left. Photograph: Pool/Reuters

A United Nations panel has decided that Julian Assanges three-and-a-half years in the Ecuadorian embassy amount to arbitrary detention, the Guardian understands, leading his lawyers to call for the Swedish extradition request to be dropped immediately.

The WikiLeaks founder sought asylum from Ecuador in June 2012 to avoid extradition to Sweden to face questioning over rape and sexual assault allegations, which he denies.

The panels findings were disclosed to the Swedish and British governments on 22 January, and will be published on Friday morning. Their judgment is not legally binding but can be used to apply pressure on states in human rights cases.

Related: Julian Assange's embassy showdown: what it means and what happens next

Assanges Swedish lawyer, Per Samuelson, said if the working group found in his favour, there is only one solution for Marianne Ny [the Swedish prosecutor seeking Assanges extradition], and that is to immediately release him and drop the case. Samuelson added: If he is regarded as detained, that means he has served his time, so I see no other option for Sweden but to close the case.

Assanges lawyers also demanded assurances from the UK that he would not be arrested and subjected to potential extradition to face potential prosecution over WikiLeaks publishing activities.

The British Foreign Office said it would not pre-empt the panels findings, but said in a statement: We have been consistently clear that Mr Assange has never been arbitrarily detained by the UK but is, in fact, voluntarily avoiding lawful arrest by choosing to remain in the Ecuadorian embassy.

An allegation of rape is still outstanding and a European arrest warrant in place, so the UK continues to have a legal obligation to extradite Mr Assange to Sweden.

Anna Ekberg, a spokesperson for the Swedish foreign ministry, said it would not comment before the formal publication on Friday. The Swedish prosecutor is currently on holiday and unavailable for comment, a spokesman said.

In a statement issued by WikiLeaks on Twitter, Assange said he would voluntarily walk out of the embassy on Friday and accept arrest as there is no meaningful prospect of further appeal.

However, should I prevail and the state parties be found to have acted unlawfully, I expect the immediate return of my passport and the termination of further attempts to arrest me.

Neither Sweden nor the UK will be compelled to take any action, but a source familiar with the working group said that if the governments choose to ignore the decision, it could make it difficult for them in future to bring pressure on other countries over human rights violations.

Assange had appealed to the UN panel claiming that his stay in the embassy was arbitrary because he had been unable to exercise his right to asylum, arguing: The only way for Mr Assange to enjoy is right to asylum is to be in detention. This is not a legally acceptable choice.

He also argued that UK law had been changed since 2012, which meant if arrested today he would no longer be liable to extradition under the European Arrest Warrant.

Melinda Taylor, an Assange legal spokeswoman, said the Australian had not yet been formally informed by the panel of its findings, but if it finds that the standard for arbitrary detention is met, we would expect his release and compensation.

In addition to Sweden dropping the extradition request, she called for the UK to return Assanges passport and give him assurances that he would not be subject to arrest for a potential further extradition request by the US.

Assange and WikiLeaks have been the subject of a secret grand jury investigation in Virginia that has been looking into whether to prosecute them over the US cable disclosures, and the Australian fears that he could become immediately subject to a second extradition process even if Sweden drops its inquiry.

If one of the orders is that he should be released and his liberty should be assured, we would obviously look to the UK to make sure that it is effective and not illusory, that its not just liberty for five seconds, but liberty that is meaningful, Taylor said.

British police ended the costly 24-hour guard of the Knightsbridge embassy last October, but the building remains under covert surveillance. A Metropolitan police spokesman said on Thursday: The operation to arrest Julian Assange does continue and should he leave the embassy the MPS will make every effort to arrest him.

Sweden and Ecuador have been locked in lengthy negotiations over arrangements to allow Swedish prosecutors to interview Assange in the embassy. Ecuadors foreign minister said last month that the country would allow access for questioning, but Sweden later said its request had been rejected on formal grounds. It is considering whether to submit a fresh request.

Samuelson said his client still hoped to clear his name. This does not mean that the question of interrogation will be over. We still want an interrogation to take place so that Mr Assange can clear his name and show everyone that he is innocent.

The difference is that he will no longer be in custody in absentia and thus be able to use his asylum outside of the embassy. If Assange is regarded as detained he has already served the time so to speak so Marianne Ny should drop the case altogether.

Last August Sweden dropped part of its investigation into Assange after the statute of limitations on allegations of sexual assault expired. It is still seeking to interview him on one outstanding allegation of rape. The accusations were made by two women in Sweden in 2010, but no charges have been brought.

Related: Timeline: Julian Assange and Sweden's prosecutors

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