The fugitive Catalan leader Carles Puigdemont has welcomed a UN report on the detention of three fellow separatists, which he said confirmed they were “political prisoners”.

The former Catalan vice-president Oriol Junqueras and the Catalan civil society group chiefs, Jordi Cuixart and Jordi Sànchez, arrested over their alleged role in the failed regional independence bid, have been in custody since late 2017 and are among 12 regional leaders currently on trial in Madrid.

A copy of the report, seen by the Guardian, concludes their detention was “arbitrary”, adding: “The working group considers that, given the circumstances of the case, the correct solution would be to free Cuixart, Sànchez and Junqueras immediately and offer them the right to obtain compensation and other forms of reparation in accordance with international law.”

Puigdemont, reacting to the as-yet unpublished findings by the UN working group on abitrary detention (UNWGAD), called on the new socialist government in Madrid to release all of the jailed Catalan activists immediately.

“This report is a clear condemnation of Spain’s breach of their fundamental rights,” the former Catalan president said before he flew out of London on Wednesday.

“It’s a shame for Spain. There’s clear abuse of detention powers. I can’t recall any case among European democracies in which the UN has expressed such clear condemnation and asked so clearly for [detainees] to be released.

“This confirms that they are political prisoners as a consequence of their illegal detention. They have lost their political rights as Spanish MPs. It’s absolutely mandatory that the Spanish authorities release them.”

The Catalan MPs were arrested – and Puigdemont fled into exile – after the Catalan independence referendum on 1 October 2017. The unilateral vote was staged in defiance of the then Spanish government of conservative prime minister Mariano Rajoy, and of the country’s constitution, which is founded on the “indissoluble unity of the Spanish nation”. The referendum law was suspended by Spain’s constitutional court three weeks before the vote.

Spain’s socialist government, led by Pedro Sánchez, has insisted the events surrounding the referendum and the Catalan parliament’s subsequent unilateral declaration of independence were judicial rather than political matters, and said it could not intervene.

The UNWGAD investigates arbitrary detentions which are alleged to be in breach of the Universal Declaration of Human Rights. It has no direct powers to enforce its decisions and relies on states to enact its decisions. It previously found in 2016 that the conditions under which Julian Assange, the Wikileaks co-founder, sought refuge in the Ecuadorian embassy in London amounted to “arbitrary detention”.

Puigdemont, who was elected an MEP last week, said he hoped Spain would respect the report. “Spain must explain to the international community how they will respect human rights. [The government] has a chance now to prove its commitment to international agreements and treaties.”

Ben Emmerson QC, the British barrister representing three of the detained Catalan MPs, said: “This decision should mark a turning point in Spanish policy towards Catalonia. Spain is acting in flagrant violation of international law, and it had been called out by the UN body with ultimate authority on the question of arbitrary detention.

“If Spain does not immediately release all the political prisoners, and sit down at the negotiating table, the government in Madrid will soon find itself struggling against the tide of international public opinion.

“The Spanish government needs to shake off the authoritarian instincts that are a hangover from Franco’s enduring dictatorship, and move very rapidly into the 21st century. Modern democracies do not lock up democratically elected politicians simply because they argue for independence. That is the hallmark of repressive regimes, and has no place in the European democratic order.”

Puigdemont said he wanted to take his seat in the European parliament but Spain needed to lift the threat of arrest against him. He was detained last year in Germany on an international arrest warrant issued by a judge in Spain. He said while he felt free to travel throughout Europe, apart from Spain, he “can’t return [to Catalonia] without the danger of being arrested”.

The Spanish government has not replied to his request to re-enter the country without being arrested.

He said he would campaign in the European parliament for “the right of self-determination in order to prevent and resolve conflicts and also to support democracy against the rise of populism”.

Puigdemont added he had been in contact with the pro-independence Scottish National party in the past and hoped to work with it in the EU parliament.

Carlos Bastarrache, the Spanish Ambassador to London, demanded the UN conduct an audit of how the report came to be compiled and leaked, claiming “rules of procedure had not been respected”.

Speaking at a briefing in London, he described the report as “a clear interference in the Spanish courts of justice with the clear objective of continuing to attack the judicial system in Spain.”

He said the report itself was marked by a lack of rigour and based on a series of false premises including the legality of the referendum. He added there had been no restrictions on freedom of expression, as the report claimed.

Additional reporting by Patrick Wintour