Despite losing the appeal, Assange's lawyers are given time to consider a challenge to the judgment

Julian Assange's fight against extradition to Sweden may stagger on to a second round at the supreme court after he was granted permission to submit fresh arguments.

Despite losing by a majority of five to two, his lawyers have been given 14 days to consider whether to challenge a central point of the judgment on the correct interpretation of international treaties.

The highly unusual legal development came after the supreme court justices decided that a public prosecutor was a "judicial authority" and that therefore Assange's arrest warrant had been lawfully issued.

Assange, who is wanted in connection with accusations of sexual assault and rape in Sweden, was not in court; there was no legal requirement for him to be present. According to his solicitor, Gareth Peirce, he was stuck in central London traffic and never made it to the court in Westminster. Assange denies the accusations.

The obscure but potentially pivotal issue raised by Dinah Rose QC, Assange's barrister, relates to Article 31.3 of the Vienna Convention on the Law of Treaties. It states that treaties can be interpreted bearing in mind the "subsequent practice" of their application.

Assange's lawyers believe that several of the justices on the seven-judge panel relied on that principle of subsequent practice in deciding that a public prosecutor had become accepted across Europe as a judicial authority.

Significantly, the court appears prepared to consider whether arguments about the Vienna Convention were raised sufficiently during the hearing. Because of security concerns the judgment was not circulated beforehand, so Assange's lawyers did not receive it in time to make representations at an earlier stage.

His lawyers may also, concurrently, begin the process of appealing against the judgment to the European court of human rights in Strasbourg. According to the supreme court, the Crown Prosecution Service cannot start extraditing Assange until 13 June at the earliest.

"The majority of the judges believe that parliament was seriously misled when it approved the European arrest warrant system," said Peirce.

"Parliament thought a 'judicial authority' meant a judge or court but the majority of supreme court judges based their decision on what is the practice in Europe and decided it on the basis of the Vienna Convention, which was never argued before the court."

The two judges who found in Assange's favour were Lord Mance and Lady Hale, who said that the court should not construe a UK statute contrary to the evidence of what parliament thought it was doing at the time.

Even though another judge, Lord Brown, has retired – at the age of 75 – since the original hearing, he may yet be called back to ensure there are an uneven number of justices on the panel if the court has to consider an emergency application from Assange.

This is the first time the supreme court has agreed to receive a challenge to one of its rulings. It happened in the past when the House of Lords was the country's highest court.

Outside the supreme court, supporters of the Wikileaks founder were holding placards declaring "Free Assange" and "Free Bradley Manning", referring to the US serviceman who is accused of sending diplomatic and security cables to the site. "Revealing the truth about war is no crime," another poster said.

The veteran Australian journalist John Pilger, who has been supporting Assange, said: "I don't think this judgment is a blow. We are disappointed but it came so close. Three of the judges [who found against Assange] were tipping in our favour.

"There was a consensus [on the bench] that parliament had been misled on this law. The court has now agreed to allow Julian Assange's legal team to go back and reconsider this. This case moves in mysterious ways and we are about to move into another mysterious stage of this whole unnecessary process."

Jago Russell, chief executive of Fair Trials International which monitors the use of European extradition warrants, said: "[This] ruling takes Julian Assange one step closer to being extradited to Sweden. Although Sweden is rightly proud of its justice system, its overuse of pre-trial detention means that, if extradited, he is likely to be imprisoned and placed under extremely restrictive conditions."

The Liberal Democrat European justice & human rights spokeswoman, London MEP Sarah Ludford, said: "I fully support reform of the European arrest warrant to stop misuse for minor cases or for the investigation stage and to stop people being whisked off to a foreign jail long before a trial is genuinely imminent."