The Swedish authorities announced on Monday that they would reopen an investigation into a rape allegation against Julian Assange, the WikiLeaks founder, who is serving a prison term in Britain for jumping bail as the United States seeks his extradition over accusations that he tried to assist in a huge breach of classified data.

The United States has already begun trying to extradite Mr. Assange, an effort that was expected to be prolonged and complex even before the announcement in Stockholm on Monday.

British officials will determine which case takes precedence, Swedish prosecutors said, adding that if Mr. Assange were eventually extradited to Sweden, he could not be sent to the United States without the consent of Britain.

The investigation stems from an accusation in August 2010 made by a Swedish woman who said Mr. Assange had sexually assaulted her.