The WikiLeaks founder, Julian Assange, has lost a high court battle to prevent the US government expanding the grounds for its appeal against an earlier refusal to allow his extradition to face charges of espionage and hacking government computers. On Wednesday, judges said the weight given to a misleading report from Assange’s psychiatric expert that was submitted at the original hearing in January could form part of Washington’s full appeal in October. Sitting in London, Lord Justice Holroyde said he believed it was arguable that Judge Vanessa Baraitser had attached too much weight to the evidence of Prof Michael Kopelman when deciding not to allow the US’s appeal. The expert had told the court he believed Assange would take his own life if extradited. But he did not include in his report the fact that Assange had fathered two children with his partner while holed up in the Ecuadorian embassy in London – a fact Assange later used in support of his bail application. Clair Dobbin QC, for the US, argued the expert misled Baraitser, who presided over the January hearing. Edward Fitzgerald QC, representing Assange, told the court Baraitser, having heard all of the evidence in the case, was in the best position to assess it and reach her decision. He said Kopelman’s report was given long before any court hearing and against a background of concern for the “human predicament” in which Assange’s partner, Stella Moris, found herself at the time. Delivering the latest decision, Holroyde said it was “very unusual” for an appeal court to have to consider evidence from an expert that had been accepted by a lower court, but also found to have been misleading – even if the expert’s actions had been deemed an “understandable human response” designed to protect the privacy of Assange’s partner and children. The judge said that, in those circumstances, it was “at least arguable” that Baraitser erred in basing her conclusions on the professor’s evidence. “Given the importance to the administration of justice of a court being able to reply on the impartiality of an expert witness, it is in my view arguable that more detailed and critical consideration should have been given to why [the professor’s] ‘understandable human response’ gave rise to a misleading report.” The US government had previously been allowed to appeal against Baraitser’s decision on three grounds – including that it was wrong in law. Assange’s legal team had described the grounds as “narrow” and “technical”. The two allowed on Wednesday were additional. During Wednesday’s hearing, the US government argued Assange, 50, was not “so ill” that he would be unable to resist killing himself if extradited – challenging Baraitser’s ruling that the US authorities could not “prevent Assange from finding a way to commit suicide” if he was extradited. Dobbin said the US government would seek to show that Assange’s mental health problems did not meet the threshold required in law to prevent extradition. She told the court: “It really requires a mental illness of a type that the ability to resist suicide has been lost. Part of the appeal will be that Assange did not have a mental illness that came close to being of that nature and degree.” Holroyde said he would not ordinarily have allowed this to form part of the appeal on its own merits alone. But he said it must be taken in the context of the broader grounds allowed and could be argued at the full hearing. Assange appeared at the hearing via video link from Belmarsh prison, wearing a dark face covering and a white shirt, with what appeared to be an untied burgundy tie draped around his neck.