WikiLeaks founder Julian Assange can be extradited to Sweden to face rape and sexual assault claims, a British judge ruled on Thursday after dismissing arguments that he would face an unfair trial.

Lawyers for Assange immediately said they would appeal against the decision and a legal battle through Britain's appeal courts could postpone the 39-year-old Australian's ultimate fate for months.

"I must order that Mr Assange be extradited to Sweden," judge Howard Riddle said in a decision at the top security Belmarsh Magistrates court in Southeast London.

Assange, wearing a dark suit and tie, sat stony-faced in the dock as the decision was read out.

Celebrity backers including socialite Jemima Khan and rights campaigner Bianca Jagger were also at the court along with around 100 journalists from around the world.

Former computer hacker Assange says the claims against him, made by two women he met during a seminar organised by the whistleblowing website in August 2010, are politically motivated because of the work of WikiLeaks.

Assange rocked the world's diplomatic institutions and infuriated Washington last year when WikiLeaks began releasing more than 250,000 secret diplomatic cables sent by US embassy staff.

It has also published sensitive data on the wars in Iraq and Afghanistan.

During three days of hearings held earlier in the month, Assange's legal team argued that the Swedish prosecutor had no power to issue a European arrest warrant and that the allegations did not amount to extradition offences.

Assange's lawyers said he would not face a fair trial as some evidence would be held behind closed doors and that it was possible he would be re-extradited to the United States where he could face the death penalty.

Riddle, however, said that the warrant was valid and said he had to uphold the 'mutual respect and confidence this court has in our European counterparts'.

"We will be appealing," his main lawyer Geoffrey Robertson told the court after the verdict.

Assange was arrested in Britain on December 7 on an international warrant issued by a Swedish prosecutor who wanted to question him over four separate allegations of sexual assault made by the two women.

He was held in London's Wandsworth prison for nine days until he was released on bail.

He has since been staying at a friend's country mansion in eastern England under strict conditions including that he obey a curfew, wear an electronic ankle tag and report to police daily.

The judge relaxed his conditions for Thursday's hearing to allow him to spend the previous night at the Frontline media club in London.

Assange claimed his greatest fear was eventual extradition to the United States, where his lawyers argued he could be sent to the Guantanamo Bay detention facility or face the death penalty.

Australia has asked the Swedish government to ensure that Assange receives a fair trial if he is ordered to go before a Swedish court, according to government documents obtained by AFP earlier this month.

Assange also recently said his site was "significantly influential" in the fall of Tunisian leader Zine El Abidine Ben Ali, an event he said "no doubt" sparked uprisings across the Middle East.