Police said Assange had violated the terms of his bail, which include an overnight curfew, and “is now subject to arrest under the Bail Act.” Police officers were stationed on Wednesday outside the Edwardian apartment block that houses the small South American country's London embassy.
The Foreign Office said that as long as Assange remains inside, he is “beyond the reach of police.”
“We will seek to work with the Ecuadorean authorities to resolve this situation as soon as possible,” it said in a statement.
The 40-year-old Australian took refuge in the mission on Tuesday, and said he was seeking political asylum in Ecuador, whose leftist President Rafael Correa has previously offered words of support.
Ecuador says it is considering the request.
Assange was arrested in London in December 2010 at Sweden's request. Since then he has been fighting extradition to the Scandinavian country, where he is wanted for questioning over alleged sexual assaults on two women in 2010.
He denies the allegations, and claims extradition could be a first step in efforts to remove him to the United States, where he claims to have been secretly indicted over his disclosure of 250,000 State Department cables.
Some legal experts said they were mystified by the reasoning behind Assange's dramatic asylum bid. But human rights lawyer Helena Kennedy, a former member of Assange's legal team, said he could be planning to bargain with Sweden for assurances that he would not be handed over to the US.
Britain's Supreme Court last week said Assange could be extradited to Sweden in about two weeks' time, rejecting his argument that a European arrest warrant issued by Swedish prosecutors for his extradition was invalid.
The only recourse left to him in the courts is an appeal to the European Court of Human Rights.
Assange, who has not been charged with any offense in Sweden and denies any wrongdoing, has argued that the case is politically motivated because the release of documents on his website has angered the United States.