The offence carries a maximum penalty of one year in prison.
Earlier this month the UK government refused to grant Mr Assange diplomatic status and called on him to leave the embassy to “face justice”.
He has long feared that if he leaves the embassy he could be sent to the US to face trial over the leaking of hundreds of thousands of secret US military and diplomatic documents.
WikiLeaks said on Tuesday that Assange could face life in prison in the US for a variety of charges including espionage, conspiracy and theft.
The group says US grand jury proceedings against Assange began as early as 2010, but it is not publicly known if he faces a US indictment because of the strict secrecy rules governing US grand jury actions.
Several prominent US officials have expressed an interest in prosecuting Assange, and there has been a US investigation into WikiLeaks’ activities.
British officials would be expected to take Assange into custody if there was a US indictment and extradition request.
Ecuador recently granted him citizenship and asylum in an attempt to resolve the political impasse over his continued presence in the UK. It had tried unsuccessfully to persuade British officials to give Assange diplomatic status, which might have made it possible for him to leave Britain even if he was sought by US officials.