Forensic experts have failed to find crucial DNA evidence in the sexual assault case against Julian Assange, a British newspaper has reported.

In a 100-page document shown to lawyers for the WikiLeaks founder, Swedish police made a case for the 41-year-old's extradition to Stockholm for questioning.

The report said staff at two forensic laboratories were unable to find conclusive evidence of Assange's DNA on a torn condom provided by one of two women who claim to have been assaulted in August 2010.

However, the same analysts have found DNA believed to belong to Assange on a condom from a second woman, The Mail on Sunday reported.

Assange denies any wrongdoing and said sex with the two women was consensual.

He remained in London's Ecuadorean embassy in a bid to avoid Swedish extradition, which he insisted would lead to him being handed to authorities in the US, where the actions of his website are under investigation.

The Swedish police report said one woman, now aged 33, claimed she was repeatedly molested by Assange at her flat in Stockholm.

She said he deliberately broke a condom before wearing it to have unprotected sex with her against her will.

Scientists were unable to find Assange's DNA on the condom and his lawyers suggested that was because a fake one may have been submitted, the newspaper said.

Assange, who had been granted asylum by Ecuador, was yet to be charged with any offence.

- AAP