Julian Assange has been blocked from seeing evidence in his extradition case because his lawyers cannot get sufficient access to him, a court has heard.

The WikiLeaks founder, 48, appeared at Westminster magistrates court by video link on Friday for a hearing to extend his detention in Belmarsh prison, in south-east London.

He is being held in the high-security jail before a full hearing in February when he will fight extradition to the US, where he faces 18 charges including conspiring to commit computer intrusion.

Assange is accused of working with the former US army intelligence analyst Chelsea Manning to leak hundreds of thousands of classified documents.

Assange appeared uncomfortable as he sat waiting for the hearing to start, clenching his hands together before putting them inside the sleeves of his grey sweater.

He spoke to confirm his name and date of birth and to clarify he was Australian, after the court’s legal adviser mistakenly suggested he was a Swedish national.

The court heard that his lawyers had made a request to the judge, complaining about a lack of access to their client behind bars.

Gareth Peirce, defending Assange, said the legal team were struggling to prepare documents for the case as Assange had no access to the evidence.

“Without Mr Assange’s knowledge, some of it is recently acquired evidence, some of it is subject to months of investigation not always in this country, of which he is unaware because of the blockage in visits,” she said.

“Despite our best efforts, Mr Assange has not been given what he must be given, and we are doing our utmost to cut through this.”

Peirce said the governor of Belmarsh had prioritised family visits over legal visits, and she asked the judge to step in. But the district judge, Vanessa Baraitser, said she had no jurisdiction over the Prison Service.

“Can I make it clear that I have no desire to stand in the way of any lawyer having proper access to their client and it’s in the interest of justice that they do,” the judge said. “What I can do and say is to state in open court that it would be helpful to this extradition process that Mr Assange’s lawyers have the access to their client.”

Assange’s lawyers have previously complained that he had been given access to an unsuitable computer in prison.

Last month more than 60 doctors warned in an open letter addressed to the home secretary, Priti Patel, that Assange could die in prison without urgent medical care.

The medics, from the UK, Australia, Europe and Sri Lanka, expressed “serious concerns” about Assange’s fitness to stand trial.

He was jailed for 50 weeks in May for breaching his bail conditions after going into hiding in the Ecuadorian embassy in London to avoid extradition to Sweden over sex offence allegations, which he has always denied. Last month WikiLeaks welcomed a decision by Swedish authorities to drop a rape investigation.

Assange has been in custody since he was removed from the embassy in April. At a hearing in October he appeared to struggle to say his own name, telling Westminster magistrates court: “I can’t think properly.”

He will next appear in court by video link on 19 December for a case management hearing.