Swedish prosecutors have dropped a preliminary rape investigation into WikiLeaks founder Julian Assange.

Key points: This was the second time the Swedish investigation had been open; it previously ran from 2010 until 2017

This was the second time the Swedish investigation had been open; it previously ran from 2010 until 2017 Assange has repeatedly denied the allegations levelled at him

Assange has repeatedly denied the allegations levelled at him The WikiLeaks founder is currently being held in a UK prison awaiting a US extradition hearing

The Swedish Prosecution Authority said the corroborating evidence had weakened considerably "due to the long period of time that has elapsed since the events in question".

The investigation was first opened after the alleged incident in August 2010 but closed in 2017, before being reopened earlier this year when Assange was arrested after being dragged out of the Ecuadorian embassy in London.

While Assange was in the embassy, the statute of limitations ran out on investigating all but one of several Swedish sex crime complaints originally filed by two women.

Deputy Chief Prosecutor Eva-Marie Persson reopened the remaining case after Assange left the embassy, but she said during a press conference in Stockholm on Tuesday the passage of time meant there was not enough evidence to indict Assange.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 34 seconds 34 s Ms Persson said the time elapsed since Assange was first accused led to her decision.

"After conducting a comprehensive assessment of what has emerged during the course of the preliminary investigation I then make the assessment that the evidence is not strong enough to form the basis for filing an indictment," she said.

"Nine years have passed.

"Time is a player in this decision."

Assange had repeatedly denied the allegations, calling them part of a plot to discredit him and secure his eventual transfer to the United States.

He is being held in a British jail pending a hearing in February on extradition to the United States, which wants the 48-year-old Australian over 18 criminal counts including conspiring to hack government computers and violating an espionage law.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 50 seconds 50 s Julian Assange arrested at Ecuadorian embassy in London

He was removed from the Ecuadorian embassy in London in April this year after spending almost seven years holed up there to avoid extradition on the Swedish allegations.

Since then, he has served a British jail sentence for skipping bail.

Extradition case looming

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WikiLeaks editor-in-chief Kristinn Hrafnsson welcomed the news the inquiry had been dropped.

"Let us now focus on the threat Mr Assange has been warning about for years: the belligerent prosecution of the United States and the threat it poses to the First Amendment," he said in a statement posted by WikiLeaks.

The US accuses Assange of conspiring with former US Army intelligence analyst Chelsea Manning to crack passwords to gain access to the information, which was subsequently posted on WikiLeaks.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 2 minutes 12 seconds 2 m 12 s Who is Julian Assange and what is Wikileaks?

Assange's Swedish lawyer, Per Samuelson, said as far as he was aware British lawyers had not yet been able to contact Assange in jail to inform him of the Swedish decision.

"This is the end of Assange's association with the Swedish justice system," Mr Samuelson said.

"But he is not happy with the way he's been treated.

"He lost faith in the Swedish justice system years ago."

Decision could be appealed

Elisabet Massi Fritz, lawyer for the accuser, told Reuters in a text message that she and her client would discuss whether to request a review of the decision to drop the case.

The right decision would have been to interrogate Assange in London and then charge him with rape, she said.

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"After today's decision my client needs time to process everything that has happened over these nine years in order to be able to move on with her life," she said.

The decision by the Swedish prosecutor heads off a potential dilemma for the British courts, which might otherwise have had to decide between competing US and Swedish extradition requests.

'Nine years of torture'

Julian Assange's father John Shipton said his son had been through "nine years of ceaseless psychological torture."

John Shipton says his son is fighting for his life. ( ABC News: Bridget Brennan )

He too was unsure whether his son had heard the news from Sweden.

"He is not in the best condition but still fighting for his life so that we won't be sent to the United States never to return," Mr Shipton told the ABC.

"It now demonstrates that Julian and his advisers were correct, the United States made every effort to drag him to America and give him a life sentence, 175 years in jail."

Mr Shipton said several Australian MPs had been "terrific" by speaking out against extraditing Assange to the United States.

He said he had asked the Australian Government and Foreign Minister Marise Payne to make representations to the British authorities about Assange's claim he had not had access to materials to defend himself in court.

ABC/Reuters