The judge said Bellfield, who was already in prison for two other murders, will never be released.
Jurors had been deliberating whether Bellfield was responsible for trying to abduct 11-year-old Rachel Cowles the day before he snatched Milly in March 2002.
But they were discharged without reaching a verdict because the judge said there was too much publicity of the case which "should not have been placed before the jury".
The judge has said Bellfield will not be retried on the count of attempted kidnap.
Milly went missing as she was walking home from school after leaving Walton-on-Thames station in Surrey a day later.
The treatment of her parents during the trial has been described as "appalling" by the Commissioner for Victims and Witnesses.
Both Bob Dowler, 59, and his teacher wife Sally, 51, broke down while giving evidence when the defence probed into their lives and suggested their daughter may have run away because she was unhappy.
Louise Casey said of the Dowlers' experience in court: "We can not let this continue."
She said theirs was not an isolated case and warned that many people let crimes go unpunished because they do not feel able to report them or give evidence.
Focus has also been thrown on the original investigation by Surrey Police, which missed chances to catch Bellfield before he carried out other murders.
During the trial it emerged that police knocked on the door of Bellfield's flat in Collingwood Place, off Station Avenue, 11 times before they got an answer.
But by that time he had moved and officers did not try to trace him. Read More