WikiLeaks said on Tuesday (Jun 25) that its founder, Julian Assange, had left a British prison on Monday and flown out of the United Kingdom.

WikiLeaks announced Assange's whereabouts shortly after court documents showed he was due to plead guilty later this week to violating US espionage law, in a deal that will allow him to return home to Australia.

Assange, 52, has agreed to plead guilty to a single criminal count of conspiring to obtain and disclose classified US national defence documents, according to filings in the US District Court for the Northern Mariana Islands.

Assange left Belmarsh prison in the UK on Monday before being bailed by the UK High Court and boarding a flight that afternoon, WikiLeaks said in a statement posted on social media platform X.

"This is the result of a global campaign that spanned grass-roots organisers, press freedom campaigners, legislators and leaders from across the political spectrum, all the way to the United Nations," the statement said.

A video posted on X by WikiLeaks showed Assange dressed in a blue shirt and jeans signing a document before boarding a private jet with the markings of charter firm VistaJet.

He will return to Australia after the hearing, the WikiLeaks statement added, referring to the hearing in Saipan.

"Julian is free!!!!" his wife, Stella Assange, said in a post on X.

"Words cannot express our immense gratitude to YOU - yes YOU, who have all mobilised for years and years to make this come true."

The only VistaJet plane that departed Stansted on Monday afternoon was headed to Bangkok, FlightRadar24 data shows. A spokesperson for Assange in Australia declined to comment on his flight plans. VistaJet did not immediately respond to a request for comment.

The Australian government, led by Prime Minister Anthony Albanese, has been pressing for Assange's release but declined to comment on the legal proceedings as they were ongoing.

"Prime Minister Albanese has been clear - Mr Assange’s case has dragged on for too long and there is nothing to be gained by his continued incarceration," a government spokesperson said.