The police investigator who first interviewed two Swedish women about allegations of rape and sexual assault against Julian Assange is a friend and political associate of one of the women, a Swedish newspaper has claimed.

The female officer became friends with the woman referred to in court as Miss A through Sweden's Social Democratic party, in which both are involved, according to Expressen. The pair corresponded on the internet 16 months before the allegations were made against Assange.

Miss A commented on a Facebook update on the police officer's page as recently as 10 February, the paper said, and Miss A links to the officer's private blog from her personal page.

The paper said the officer had made anti-Assange comments on the internet.

The WikiLeaks founder is appealing against a British magistrate's decision last month to extradite him to Sweden to answer the accusations, which include an allegation of rape against another woman, Miss B. Miss A alleges Assange had sex with her without a condom, against her wishes. He has not been charged with any offence.

His legal team has argued that the Swedish judicial process is unfair and a number of those involved in the prosecution are politically motivated.

According to Expressen, Miss A and the police interrogator had internet contact in April 2009, when Miss A wrote a blog about white men "who take the right to decide what is not abusive". The officer commented that the author "puts her finger on the bottom line and speaks out", to which Miss A replied: "Hello! Thanks for the compliment. And like you say, white men must always defend the right to use abusive words. Then they of course deny that these very words are part of a system that keeps their group at the top of the social ladder."

The paper said that when another newspaper, Aftonbladet, hosted a recent webchat with Assange, the officer commented "What the heck is this! Judgment zero!" The previous day she had commented on the same page: "Way to go, Claes Borgstrom!" Borgstrom is the lawyer representing the women and a former SDP politician, who Assange's team has argued is acting from political motives.

The paper says the officer had just started her shift at Klara police station in Stockholm on 20 August when Miss A and Miss B arrived to make a complaint against Assange. It says she did not declare a conflict of interest. The police say that the officer in question did not interview Miss A and she played no further part in the investigation. On the basis of the interrogations, duty prosecutor Maria Häljebo Kjellstrand ordered Assange's arrest, a decision overturned by a more senior prosecutor. Borgstrom appealed against that decision and the case was reinstated by prosecutor Marianne Ny.Mark Stephens, Assange's lawyer, said they had been aware of the relationship, which had informed their arguments in court last month that the Swedish judicial process had been improper.

"There are a whole raft of issues like this which should cause reasonable people a bit of concern," he said. "I'm delighted that the Swedes, who objected so strongly to our criticisms of the case, have started to acknowledge that there are systemic problems in their judicial process which allow this sort of thing to happen."

Police superintendent Ulf Göranzon told Expressen he was not aware of any relationship between the two women, and would not comment on rumours.

The Swedish prosecutor's office also declined to comment, citing the ongoing extradition process in the UK.