British Home Secretary Sajid Javid told BBC Radio 4's Today that he has signed Julian Assange's extradition order.
"The final decision is now with the courts," Mr Javid said.
It is unclear whether the WikiLeaks founder will be sent to Sweden or the US.
It was earlier reported that the US had formally submitted an extradition request to the UK for the WikiLeaks founder.
Mr Assange faces an 18-count indictment that accuses him of soliciting and publishing classified information and of conspiring with former Army private Chelsea Manning to crack a Defense Department computer password.
That indictment, which includes Espionage Act charges, was issued by the Justice Department last month and is pending in a federal court in Alexandria, Virginia.
The extradition request had been expected ever since US authorities first announced a criminal case against Mr Assange.
The 47-year-old was evicted on 11 April from the Ecuadorian Embassy in London, where he had been holed up since 2012 after Ecuador granted him political asylum.
He was arrested by British police and is currently serving a 50-week sentence for jumping bail.
Sweden also seeks him for questioning about an alleged rape, which Mr Assange has denied.
Mr Assange was initially charged with a single computer crime violation on allegations that he worked with Ms Manning to crack a government password.
Some legal experts have said the additional Espionage Act charges might slow or complicate the extradition process to the extent the UK views them as political offences and therefore exempt from extradition.
The US official spoke on condition of anonymity because the official wasn't authorised to speak publicly.
Ms Manning, who spent seven years in a military prison for delivering a trove of classified information to Mr Assange before having her sentence commuted by then-President Barack Obama, has been jailed for civil contempt in Virginia after refusing to testify before a grand jury investigating WikiLeaks.
US charges against Assange to be unveiled on Friday
The United States will detail all the charges against Mr Assange when it seeks his extradition in a London court on Friday, the editor of the whistleblowers' website said on Tuesday.
"The American authorities, the Department of Justice, will present the evidence in support of their extradition demand," Kristinn Hrafnsson told reporters.
The US Justice Department confirmed on Tuesday that it had submitted a formal extradition request.
The 47-year-old Australian is not expected to attend Friday's hearing but could take part from prison via video link, although it will be largely procedural.
The "first real confrontation of arguments" in court will not be for several weeks or months, Mr Hrafnsson said.
On the run from extradition to Sweden on accusations of sexual assault, Mr Assange lived inside Ecuador's embassy to London for seven years after being granted asylum.
The US has accused Mr Assange of violating the US Espionage Act by publishing military and diplomatic files in 2010.
The 18 charges against Mr Assange reject his claim he was simply a publisher receiving leaked material - which would be protected under press freedom legislation.
Mr Hrafnsson said the charges were "very revealing about the nature of this entire case".
He said the Espionage Act was part of an "archaic legal framework... and has never been used against a publisher and a journalist".
"It's an indication of the watershed moment that we are now seeing in the attack on journalism," he said.
Detained at London's Belmarsh high-security prison, Mr Assange has been transferred to a medical unit due to concerns about his health.
He received a visit on Tuesday from his father, John Shipton, and the Chinese dissident artist Ai Weiwei, Britain's Press Association reported.
