WikiLeaks founder Julian Assange is said to have lost access to the internet and the right to receive visitors at the Ecuadorian embassy in London.
Officials announced today they were taking the measure in response to Mr Assange's recent activity on social media.
As part of an agreement between Mr Assange and the Ecuadorean government, he is not permitted to send any messages that could interfere with the South American nation's relations with other countries.
He has been living at the embassy since June 2012 to avoid extradition to Sweden to face questioning over allegations of sex crimes, which he always denied.
The Swedish authorities have dropped their investigation, but Mr Assange believes he will be extradited to the United States for questioning over the activities of WikiLeaks if he leaves the building.
Former Greek minister Yanis Varoufakis and musician Brian Eno said in a statement they had heard "with great concern" about the lost internet access and ban on visitors.
"Only extraordinary pressure from the US and the Spanish governments can explain why Ecuador's authorities should have taken such appalling steps in isolating Julian," they said.
They added that the Ecuador government had only recently granted Mr Assange citizenship, saying it must have been "leaned on mercilessly" to stop attempting to provide a diplomatic route to safety and even drive the WikiLeaks founder out of the embassy.
Foreign Office Minister Sir Alan Duncan branded Mr Assange a "miserable little worm" during a Commons debate on Tuesday, adding he should leave the Ecuadorean embassy and surrender to British justice.
Mr Assange replied: "Britain should come clean on whether it intends to extradite me to the United States for publishing the truth and cease its ongoing violation of the UN rulings in this matter.
"If it does this disgraceful impasse can be resolved tomorrow. I have already fully served any theoretical (I haven't been charged) "bail violation" whilst in prison and under house arrest. So why is there a warrant for my arrest?"