Stella Assange said the WikiLeaks founder was told he was being moved to a bare cell “for his own protection”.
He had no visits over the weekend following the announcement last Friday.
“Imagine what it’s like for him to have to process this alone,” said Mrs Assange.
John Rees, a leading member of the campaign for Mr Assange to be freed, told the PA news agency: “This is simply extrajudicial punishment.
“It’s unacceptable and it’s surely illegal. But it shows how much pressure the authorities are under to free Assange that they behave this vindictively.
“We need to redouble our efforts to stop the extradition, for Julian Assange’s sake and for the defence of a free press.”
Last week Priti Patel signed an order to extradite Assange to the US to face espionage charges.