WikiLeaks founder Julian Assange has made his first public statement since being arrested, saying the sex charges against him have only increased his determination.

Mr Assange's message from inside London's Wandsworth prison was relayed through his mother Christine Assange on Tuesday, ahead of a bail hearing at Westminster Magistrates Court.

Ms Assange wrote down her son's comments during a 10-minute phone conversation from inside the prison, Seven Network reported.

"My convictions are unfaltering. I remain true to the ideals I have expressed. This circumstance shall not shake them," he wrote.

"If anything, this process has increased my determination, that they are true and correct."