A group campaigning for the rights of sex workers has claimed the murder of a mum-of-four from Rotherham could and should have been prevented.
Alena Grlakova's body was found in a stream in the Parkgate area of the town in April 2019 - more than three months after she went missing.
In 2000, Allen was acquitted of the earlier killing. He became the subject of an undercover Humberside Police investigation when he moved back to the area 10 years later.
The force had been hoping to secure a ruling against 'double jeopardy' the principle that you can't be tried twice for the same offence unless there is new and compelling evidence, but its request was denied.
Niki Adams, English Collective of Prostitutes speaks to ITV News
The English Collective of Prostitutes, which campaigns for the rights of sex workers, believes the decision is an example of sex workers' lives not being treated equally.
Niki Adams, from the group, said: "This does feel like an avoidable tragedy yet again other women have gone on to lose their lives because the violence of a particular man was not addressed properly at the time and that is a pattern that we have seen over and over again.
"It's a pattern that starts with the deprioritising of sex workers' safety."
The law changed in 2003 to allow the Crown Prosecution Service to apply to the Court of Appeal to allow a retrial for murder where new and compelling evidence had come to light after an acquittal.
A spokesperson for the Crown Prosecution Service said: "Based on the law in 2011, the CPS advised the police that it was unlikely that admissions to a police undercover officer alone would be sufficiently compelling to persuade the Court of Appeal to order a retrial.
"The decision was made knowing that applying to the Court of Appeal to overturn a conviction is a once-only option. If it failed there could be no further attempts.
"Since 2011 the law in relation to double jeopardy murders has evolved, and after the death of Alena Grlakova a thorough and detailed review was carried out of all new lines of evidence which had emerged following the acquittal in 2000."