LONDON—Britain’s Foreign Office says it has delivered a letter to Ecuador’s embassy where WikiLeaks founder Julian Assange has been holed up to avoid extradition to Sweden.

The Foreign Office declined to comment Friday on the content of the communication. The letter was delivered on Thursday.

Ecuador’s President Rafael Correa granted Assange asylum last week. But while Assange remains out of reach of British authorities while he is inside the embassy, he has no clear way of getting to Ecuador. Britain insists that if he steps outside, he will be detained and sent to Sweden, where he is wanted over allegations of sexual assault.

The standoff has the potential to drag on for weeks, months, or even years; the world’s media has followed every move in the increasing convoluted saga.