(CNN) -- The Supreme Court of the United Kingdom on Thursday dismissed an application filed by an attorney for WikiLeaks founder Julian Assange seeking to reopen his appeal against extradition to Sweden.
The application was Assange's last option in the British courts. Britain's Crown Prosecution Service has previously said if the court dismissed Assange's appeal, his only further remedy is to apply immediately to the European Court of Human Rights, and Assange's attorneys have vowed to do so.
The appeal itself would be a rarity, as the court's decisions are supposed to be final in Britain.
The application by Dinah Rose, Assange's attorney, was "without merit," the seven British justices unanimously agreed, according to a court statement Thursday.
Assange has been fighting for a year and a half against being sent to Sweden for questioning about accusations of sexual abuse. Two women accused him in August 2010 of sexually assaulting them during a visit to Sweden in connection with a WikiLeaks release of internal U.S. military documents.
WikiLeaks' work is not at issue in the extradition matter or the Swedish allegations against Assange.
Assange has not been charged with a crime, but Swedish prosecutors want to question him about allegations of "unlawful coercion and sexual misconduct including rape," according to a Supreme Court document.
Assange has been under house arrest in Britain since December 2010. He has maintained his innocence and claims the allegations against him are politically motivated. He fears that if he is extradited to Sweden, authorities there could hand him over to the United States, where he then could be prosecuted for his role in the leaking of classified documents.
WikiLeaks on Twitter called the court's behavior "odd, dangerous and wrong."
Earlier, the organization tweeted that Assange was to be extradited and asked for donations to his defense fund.
Assange's lawyers fought the extradition on appeal via a legal technicality involving the arrest warrant, claiming the "European Arrest Warrant" issued for him was invalid because it was issued by a Swedish prosecutor and not an independent and impartial judicial authority.
Two women Assange had sexual relations with in Sweden in August 2010 subsequently went to police, who took down their complaints, according the Britain's Supreme Court. Police then interviewed Assange.
The WikiLeaks founder left Sweden "in ignorance of the fact that a domestic arrest warrant had been issued for him," according to the UK high court. A Swedish court granted a warrant for his "detention for interrogation," and Swedish prosecutors issued the European Arrest Warrant for his detention in Britain.
WikiLeaks, which facilitates the anonymous leaking of secret information, has published some 250,000 confidential U.S. diplomatic cables, causing embarrassment to the government and others.
It has also published hundreds of thousands of classified U.S. documents relating to the conflicts in Iraq and Afghanistan.
But the organization has come under financial pressure, leading Assange to announce that WikiLeaks was temporarily stopping publication to "aggressively fund raise" in order to stay afloat.
An announcement at the top of WikiLeaks' home page reads: "We are forced to put all our efforts into raising funds to ensure our economic survival."
During his wait for the Supreme Court to rule on his extradition in May, Assange has hosted a talk show on Russian TV. "The World Tomorrow" appears on the Kremlin-funded, pro-Russian network Russia Today. He hosts it from the Suffolk, England, mansion where he is under house arrest, with an electronic bracelet monitoring his movements.
He has interviewed controversial figures at odds with the U.S. government, including Hassan Nasrallah, the leader of Hezbollah, which the United States considers a terrorist organization, and Ecuador's president, Rafael Correa, who railed against the United States in his interview with Assange.
CNN's Per Nyberg and Atika Shubert contributed to this report.