Mr Assange has been inside the Embassy of Ecuador in London since last week seeking political asylum as part of his bid to avoid extradition to Sweden, where he is wanted for questioning about alleged sex offences.
It is understood that officers from the Metropolitan Police's extradition unit delivered a note to the embassy this morning saying Mr Assange has to present himself to Belgravia police station at 11.30am tomorrow.
Sources said a letter was also delivered for Mr Assange.
Mr Assange suddenly arrived at the Embassy on Tuesday in a dramatic twist to his fight to avoid extradition to Sweden where he wanted for questioning about alleged sex offences.
The Foreign and Commonwealth Office confirmed in a statement that the Australian was ''beyond the reach of the police'' while he remains in the building.