WikiLeaks founder Julian Assange's wife Stella said she was "elated" that he will be a "free man" after a US judge signs off on his landmark plea deal on Wednesday.

Stella Assange said the end of his years-long legal drama that saw him board a plane from London to Bangkok on Monday had been a "whirlwind of emotions".

"I'm just elated. Frankly, it's just incredible," the South African-born rights campaigner told BBC radio.

"We weren't really sure until the last 24 hours that it was actually happening."

Assange was on Monday freed on bail from a high-security prison in southeast London ahead of a plea hearing expected to take place in remote US territory.

He had been held in jail 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 defence information, according to a document filed in court in the Northern Mariana Islands in the Pacific, where the hearing is expected to take place.

Stella Assange said the plea deal involved her husband pleading guilty to the single charge.

"The charge concerns the Espionage Act and obtaining and disclosing national defence information," she said.

"The important thing here is that the deal involved time served that if he signed it, he would be able to walk free," she added.

Extradition call to be withdrawn

A charter plane flew the 52-year-old Australian publisher from London to Bangkok, where it made a scheduled stop to refuel.

From there it is set to fly to Saipan, the capital of the US territory where Assange is due in court on Wednesday morning.

"What there is, is an agreement in principle between Julian and the Department of Justice, and that has to be signed off by a judge in these Northern Mariana territories... in the Pacific Ocean where he is going to be headed," she said.

"He will be a free man once it has been signed off... and that will happen some time tomorrow."

Reporters Without Borders welcomed Assange's release as a "long overdue victory for journalism and press freedom".

"He never should have spent a single day deprived of his liberty for publishing information in the public interest," said RSF's director of campaigns Rebecca Vincent. 

"Nothing can undo the past 13 years, but it is never too late to do the right thing, and we welcome this move by the US government."

A High Court order for Assange's appeal against extradition that was due to be heard next month said a plea agreement between the two parties was signed last Wednesday.

The Crown Prosecution Service, which decides on whether to take cases to court in England and Wales and also handles extradition requests, said a bail hearing was held in private, at Assange's request, last Thursday.

He left the jurisdiction of England and Wales at 6:36 pm (1736 GMT) on Monday, it added.

Stella Assange, meanwhile, told Britain's domestic Press Association news agency that her husband would be required to pay $500,000 (£390,000) to cover the cost of the chartered flight taking him from London to Australia.