The British High Court will decide later whether it will allow the US permission to appeal after the same date, Assange's partner Stella Moris said in a tweet on Thursday.
On 4 January, British District Judge Vanessa Baraister ruled that Assange should not be extradited, saying the risk he would commit suicide in a US jail were too high.
Assange faces criminal charges for publishing classified information that was leaked to WikiLeaks by an American soldier, then known as Bradley Manning, but now, after gender reassignment surgery, known as Chelsea Manning.
New: The Court has now granted Julian's team an extension until March 29th to reply to the US appeal grounds/cross-appeal.— Stella Moris #DropTheCharges (@StellaMoris1) February 18, 2021
The High Court will decide whether it will allow the US permission to appeal after March 29th.
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After saying it would be doing so, the new administration confirmed on 10 February that it would be continuing its bid to extradite Assange.
In a statement, Moris said: "The Biden Administration will soon appoint its new attorney-general and this will be an important moment to raise the pressure on the Biden administration to live up to its commitments to defend press freedom and drop the charges against Julian.
"The Obama administration, of which Biden was vice-president, decided not to pursue charges against Julian because it recognised that to do so would be a wider attack on press freedom.
"It was the Trump administration which charged him and pursued the extradition as part of its war against journalism."