Allegations against Julian Assange, the founder of Wikileaks, took an unexpected turn today when Sweden's top prosecutor announced she was reopening a rape investigation.

It is the second time the country's authorities have made a U-turn over whether the Australian-born journalist should face charges.

An arrest warrant for the founder of the whistleblowing website was initially issued on 20 August, after allegations of rape and assault were brought against him by two women who approached police simultaneously.

The warrant was abruptly rescinded within 24 hours and last week Stockholm's chief prosecutor Eva Finne said all charges against the 39-year-old would be dropped. But Ms Finne's boss, Marianne Ny, Sweden's chief prosecutor, has overruled that decision after new information reportedly came to light on Tuesday.

"There is reason to believe that a crime has been committed," she said in a statement published on the Prosecution Authority's website. "Considering information available at present, my judgement is that the classification of the crime is rape. More investigations are necessary before a final decision can be made."

There was no immediate comment from Mr Assange but his lawyer Leif Silbersky told the Swedish newspaper Expressen: "Now I'm really surprised. I thought that Eva [Finne] is a skilled and competent prosecutor who draws the correct conclusions from the material that is available to her. Now another prosecutor says that she has not done that. We are back to square one - the circus continues."

Mr Silbersky, a colourful septuagenarian defence attorney who has a reputation for taking on some of Sweden's most controversial and high-profile cases, said Mr Assange was still in Sweden. He added that the Wikileaks founder was "angry and disappointed" with the country's legal system.

Mr Assange has previously suggested that the allegations are part of a smear campaign following his website's release of thousands of secret US Army documents relating to Afghanistan.

According to the Swedish newspaper Aftonbladet, Mr Assange was questioned on Monday afternoon at a police station in Kungsholmen, Stockholm. Sources told the newspaper that he had admitted staying at one of the women's flats for a week in August and having sex with her - but denied rape.

Claes Borgstrm, a lawyer who represents both women, appealed against the decision made last week to drop the charges against Mr Assange. In a statement, he said he welcomed the decision to reopen the investigation.

"This is a redress for my clients, because they have been dragged through the mud on the internet for having 'made things up' or 'intending to frame Assange'," he said.

It is unclear whether the charges will affect Mr Assange's chances of staying in Sweden, where a number of Wikileaks' servers are based. Following the releases of the Afghan war logs, Mr Assange moved from London to Sweden and applied for residency there.

His website, which promotes leaking information to fight government and corporate corruption, is planning to release 15,000 more secret documents on the Afghan war. US officials have condemned the threat of future releases.