British Home Secretary Sajid Javid says he has approved the extradition of WikiLeaks founder Julian Assange to the United States, telling the BBC that it will be up to the courts to decide the next step.

Mr Javid told the broadcaster that the 47-year-old Australian citizen was "rightly behind bars".

AAP

"There's an extradition request from the US that is before the courts tomorrow, but yesterday I signed the extradition order and certified it, and that will be going in front of the courts tomorrow," he said.

Mr Assange's lawyers are expected to challenge Mr Javid's decision, paving the way for a lengthy appeal process through British courts.

The US government has accused Mr Assange of conspiring with former US military intelligence analyst Chelsea Manning to leak a trove of classified material in 2010.

He has been held at London's Belmarsh prison since police dragged him from the Ecuadorian embassy in London on 11 April.

He had spent seven years inside the embassy to avoid arrest.

READ MORE Assange's father in emotional jail visit as US launches extradition bid

Mr Assange was sentenced to 50 weeks in prison on 1 May after he was found guilty of breaching bail conditions related to an earlier extradition request from Sweden, which had wanted him to answer allegations of rape and sexual assault.

After visiting him at the prison hospital on Tuesday, Chinese dissident artist Ai Weiwei reported that Assange's health is "deteriorating".