"I accept that they trusted Mr Assange to surrender himself as required. I accept that they followed the proceedings and made necessary arrangements to remain in contact with him," he said.
"However, they failed in their basic duty, to ensure his surrender. They must have understood the risk and the concerns of the courts.
"Both this court and the High Court assessed that there were substantial grounds to believe the defendant would abscond, and that the risk could only be met by stringent conditions including the sureties."
Vaughan Smith, a friend of Mr Assange, addressed Westminster Magistrates Court last week on behalf of the nine people, who put up £140,000 between them.
He said all those who had offered sureties were "convinced that they have done and are doing the right thing".
The chief magistrate decided each of the backers had to pay part of the sum originally pledged, under the 1980 Magistrates Court Act.
They were as follows: retired professor Tricia David £10,000, Lady Caroline Evans £15,000, Joseph Farrell £3,500, Sarah Harrison £3,500, journalist Phillip Knightley £15,000, friend Sarah Saunders £12,000, friend Vaughan Smith £12,000, scientist Sir John Sulston £15,000 and Baroness Tracy Worcester £7,500.
