Appeal judges are to decide whether the life sentence handed out to a murderer was too lenient.
Christopher Halliwell, 48, was told by a judge last month that he must serve a minimum of 25 years' imprisonment before he could apply for parole.
But Crown Prosecution Service (CPS) Wessex has confirmed it has referred the case under the Unduly Lenient Sentence Scheme to the Attorney General, who in turn has referred it to the Court of Appeal.
Nick Hawkins, chief Crown prosecutor for CPS Wessex, said: "Christopher Halliwell was sentenced on October 19 2012 to life imprisonment, with a minimum term of 25 years for the murder of Sian O'Callaghan.
"The Crown Prosecution Service Wessex after careful consideration has referred this sentence to the Attorney General, Dominic Grieve QC MP, under the Unduly Lenient Sentence Scheme (ULS).
"Mr Grieve QC, after consideration of our submission, has made an application to the Court of Appeal to consider whether this sentence was unduly lenient."
Halliwell, a taxi driver, admitted the murder of office worker Ms O'Callaghan, 22, who was killed after she left a nightclub in Swindon in March last year.
The father of three also confessed to killing missing prostitute Rebecca Godden and even led officers to both of the women's bodies.
But a senior officer's failure to follow the rules meant the murder charge in Miss Godden's case had to be dropped.
Her mother, Karen Edwards, has now launched a campaign to change the law, while the senior investigator, Detective Superintendent Steve Fulcher, has been suspended pending an Independent Police Complaints Commission inquiry.