The US Justice Department has delivered to officials in the United Kingdom a formal extradition request for Julian Assange, making further US charges against the WikiLeaks founder unlikely.

A US official who spoke on background to discuss a sensitive matter said the request was sent on Thursday.

The United States’ treaty with Britain required that the request be sent within 60 days of Assange’s 11 April arrest at Ecuador’s Embassy in London.

The same treaty bars the United States from prosecuting Assange for any alleged crimes beyond those outlined in the extradition request, unless those acts occur after his extradition.

In an 18-count indictment filed last month, prosecutors charged Assange with violating the Espionage Act and conspiring to hack into a government computer.

Key moments for Julian Assange Show all 9 1 /9 Key moments for Julian Assange Key moments for Julian Assange The situation today Assange was arrested after Metropolitan Police officers were invited into the Ecuadorian embassy on April 11 2019. How did it come to this? Ruptly TV Key moments for Julian Assange The break Assange shows the front page of the Guardian on July 26 2010, the day that they broke the story of the thousands of military files leaked by WikiLeaks AFP/Getty Key moments for Julian Assange Wanted A warrant for Assange's arrest was issued in August 2010 for counts of rape and molestation in Sweden AFP/Getty Key moments for Julian Assange Ruling The UK's Supreme Court ruled in 2012 that Assange should be extradited to Sweden to face trial Getty Key moments for Julian Assange Sanctuary Following the ruling, Assange was given asylum by the Ecuadorian governement over fears that his human rights would be violated if he were extradited, he has since remained in the embassy in London Getty Key moments for Julian Assange A friend in Pam Friend Pamela Anderson delivers lunch to Assange at the embassy in October 2016. She has since spoken against his arrest Getty Key moments for Julian Assange Arbitrarily detained A UN panel found in 2016 that Assange had been arbitrarily detained and that he had not been able to claim his full right to asylum. It urged Sweden to withdraw the charges against him Getty Key moments for Julian Assange The cat ultimatum Last year, the Ecuadorian embassy threatened to revoke Assange's internet access unless he stopped making political statements online and started taking better care of James, his pet cat. Assange accused Ecuador of violating his rights Reuters Key moments for Julian Assange Arrest Assange was arrested on April 11 2019. Ecuador revoked his asylum status and invited the Metropolitan Police in to the embassy to arrest him. Reuters

The Justice Department did not pursue Assange for the 2017 exposure of Central Intelligence Agency hacking tools known as “Vault 7”, according to government officials, out of concern that doing so would do more damage to national security.

Joshua Adam Schulte, a former CIA employee, is accused in New York federal court of leaking that information to WikiLeaks.

A grand jury investigation of Assange has remained active in recent weeks.

Former Army intelligence analyst Chelsea Manning, whose interactions with Assange form the basis of the charges against him, remains in jail for refusing to testify before the grand jury.

Sigurdur Thordarson, a former WikiLeaks associate who became an FBI informant, says he voluntarily met with prosecutors in Virginia late last month and was asked detailed questions about Assange’s relationships with hackers.

A spokesman with the US attorney’s office in Alexandria declined to comment on Mr Thordarson’s account.

When Assange was arrested, US officials indicated that they would seek his extradition, which he has been fighting from jail in London.

He is serving a 50-week sentence for fleeing to the Ecuadorian Embassy in 2012 rather than face sexual assault charges in Sweden.

He missed a scheduled court hearing last week because of illness, his lawyers told reporters.

He is still under investigation for rape in Sweden, although a court there recently ruled that he should not be extradited.

A 12 June hearing is set for Assange in the United Kingdom.