The WikiLeaks founder was sentenced in May to 50 weeks behind bars for skipping bail and hiding out in the Ecuadorian embassy in London for seven years to avoid extradition to Sweden over sex offence allegations.
Assange entered the embassy in 2012 and stayed there until he was dramatically dragged out by police in April.
He was due to be released from HMP Belmarsh on 22 September, but a judge at Westminster Magistrates’ Court told the 48-year-old on Friday there was “substantial ground” to believe he would “abscond again”.
“I have given your lawyer an opportunity to make an application for bail on your behalf and she has declined to do so,” district judge Vanessa Baraitser told him.
“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 is now facing extradition to the US over allegations of computer hacking after the then home secretary Sajid Javid signed an order in June.
A final hearing in the case is expected in February.
Appearing in court via video-link on Friday, Assange said he didn’t really understand what the judge was telling him, but was “sure the lawyers would explain it”.
An administrative hearing is now scheduled for 11 October and a case management hearing for 21 October.