Julian Assange was released from prison Monday and has left Britain, WikiLeaks said, as he reached a landmark plea deal with US authorities that brought an end to his years-long legal drama.

"Julian Assange is free," WikiLeaks wrote on X of its founder, who had been detained in Britain for five years as he fought extradition to the United States which sought to prosecute him for revealing military secrets.

He has agreed to plead guilty to a single count of conspiracy to obtain and disseminate national defense information, according to a document filed in court in the Northern Mariana Islands in the Pacific.

Assange is scheduled to appear in the US territory on Wednesday morning local time.

He is expected to be sentenced to 62 months in prison, with credit for the five years and two months he has served in prison in Britain. This means he could return to his native Australia.

The Australian government responded that Assange's case had "dragged on for too long" and there was "nothing to be gained by his continued incarceration."

The publisher, now aged 52, was wanted by Washington for publishing hundreds of thousands of secret US documents from 2010 as head of the whistleblowing website WikiLeaks.

During his ordeal Assange became a hero to free speech campaigners around the world and a villain to those who thought he endangered US national security and intelligence sources by revealing secrets.

US authorities wanted to put Assange on trial for divulging military secrets about the wars in Iraq and Afghanistan.

The plea bargain agreement will presumably end Assange's nearly 14-year legal drama.

Assange was indicted by a US federal grand jury in 2019 on 18 counts stemming from WikiLeaks' publication of a trove of national security documents.

Extradition battle

WikiLeaks released a short video of Assange speaking with people in what appeared to be an office and then boarding a plane.

According to Thai officials, the plane carrying Assange was expected to refuel and resupply in Bangkok before carrying on to Saipan, the capital of the Northern Mariana Islands.

Assange's family expressed deep gratitude for his freedom, including his mother Christine Assange who said in a statement carried by Australian media that she was "grateful that my son's ordeal is finally coming to an end."

His wife Stella meanwhile thanked campaigners, writing on social media platform X that "words cannot express our immense gratitude."

Assange met his wife while he was holed up in Ecuador's London embassy, then married her in a ceremony in prison. They now have two young children.

Announcement of the deal came two weeks before Assange was scheduled to appear in court in Britain to appeal a ruling approving his extradition to the United States.

Assange had been detained in the high-security Belmarsh prison in London since April 2019.

He was arrested after spending seven years in Ecuador's London embassy to avoid extradition to Sweden, where he faced accusations of sexual assault that were eventually dropped.

The material he released included video showing civilians being killed by fire from a US helicopter gunship in Iraq in 2007. The victims included two Reuters journalists.

The United States has accused Assange under the 1917 Espionage Act. Supporters have warned this means he could be sentenced to 175 years in prison. 

The British government approved his extradition in June 2022. 

In the latest twist to the saga, two British judges said in May that he could appeal against his extradition to the United States.

The appeal was to address the question of whether, as a foreigner on trial in America, he would enjoy the protections of freedom of speech accorded under the First Amendment to the US Constitution.

The plea deal was not entirely unexpected. President Joe Biden had been under growing pressure to drop the long-running case against Assange.

In February the government of Australia made an official request to this effect and Biden said he would consider it, raising hopes among Assange supporters that his ordeal might end.

Emma Shortis, senior researcher at The Australia Institute think tank, told AFP that both Canberra and Washington had a "recognition that this had to end."

"There was just no way this wasn't becoming an issue for the (US-Australia) alliance," Shortis said.