The letter was delivered to the Ecuadorian embassy in London, where the WikiLeaks founder has been waiting nine days for the country's decision on his asylum request.
The Metropolitan Police served "a surrender summons on 40-year-old Assange requiring him to come to the police station on a date and time to be determined by us".
The police statement added that this is standard procedure in extradition cases and that it is the first step in the process.
Assange came to the Ecuadorian embassy after the Supreme Court of Great Britain ruled that he can be extradited to Sweden, where he is wanted on charges of rape and sexual assault on two women.
The Australian denies the accusations, which he claims were made against him for political reasons.
Assange fears that Sweden could immediately extradite him to the US, where he could be prosecuted for revealing hundreds of thousands of classified diplomatic cables.
Assange was arrested in London in December 2010 at the request of Sweden and spent the last 18 months under house arrest from where he fought extradition.
British officials said he was out of their reach as long as he was in the embassy, but that if he got out, they would immediately arrest him for breaching his bail conditions.
Ecuador said it was still considering his asylum request.
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