Swedish prosecutors have dropped their long-running investigation into sexual offences allegedly committed by Wikileaks founder Julian Assange, judging that witnesses' memories are too hazy to corroborate his accuser's account after nine years.

"After completing the investigation over the summer and autumn, it is now clear that oral testimony supporting the claimant's account has weakened," Eva-Marie Persson, Deputy Director of Public Prosecution, told a press conference in Stockholm on Tuesday afternoon.

"This is mostly due to the length of time that has passed since the events. Naturally, memories fade."

Since reopening the case in May, Ms Persson had interviewed seven witnesses but had yet to question Mr Assange himself.

The decision not to press charges comes as the 48-year-old activist remains in London's Belmarsh prison awaiting a hearing next February over possible extradition to the US, where he faces 18 criminal charges related to Wikileaks' 2010 release of thousands of classified documents detailing US military activity in Iraq and Afghanistan.

After the announcement, Nils Melzer, UN Special Rapporteur on Torture, called for an urgent investigation into Sweden's treatment of Mr Assange, arguing that it had always been "inevitable" that the investigation would be discontinued, given "its gross arbitrariness".