He would have been released from HMP Belmarsh on September 22, Westminster Magistrates' Court heard on Friday, but the 48-year-old Australian was told he will be kept in jail because of "substantial grounds" for believing he will abscond again.
The WikiLeaks founder appeared by video-link wearing a loose-fitting T-shirt.
District Judge Vanessa Baraitser told him: "You have been produced today because your sentence of imprisonment is about to come to an end.
"When that happens your remand status changes from a serving prisoner to a person facing extradition.
"Therefore I have given your lawyer an opportunity to make an application for bail on your behalf and she has declined to do so. Perhaps not surprisingly in light of your history of absconding in these proceedings.
"In my view I have substantial ground for believing if I release you, you will abscond again."
Assange was asked if he understood what was happening, and replied: "Not really. I'm sure the lawyers will explain it."
Another administrative hearing will take place on October 11 following by a case management hearing on October 21, the court heard.
The final hearing in Assange's extradition case is due in February.
He entered the embassy in 2012 to avoid extradition to Sweden, where he was wanted in connection with sexual offence allegations.
Assange spent nearly seven years living inside the Ecuadorian embassy until he was dramatically dragged out by police in April, after Ecuador revoked his political asylum.