John Worboys, a black-cab driver who is believed to be among the most prolific sex offenders in recent UK history, is set to be released from prison in a decision that has sparked widespread outrage.

The taxi driver scoured West London for lone women between 2002 and 2008, and gave them champagne laced with sedatives under the pretence that he had won lottery money. He then attacked them in his cab. Worboys was convicted of targeting 12 women, in attacks that involved sexual assault and rape. However, police fear the number of women he attacked was as high as 100.

A jury at Croydon Crown Court found him guilty of 19 offences, including rape and drugging, in 2009. He was ordered to serve a minimum of eight years in prison. Worboys has since spent 10 years in custody, including a period on remand.

Responding to Worboys' upcoming release, Nick Hardwick, the chairman of the Parole Board, apologised "unreservedly" that victims were not told that he would be freed. But Hardiwick said that he was "confident" the former cab driver would not re-offend, BBC News reported.

The decision to release Worboys has been widely condemned, with Yvonne Traynor, CEO of Rape Crisis South London, calling it "dangerous" and describing the time he spent in prison as "woefully short".

So why was Worboys released in what appears to be a shorter time than expected?

Around once a year, the Parole Board - an independent body - considers assessments by a psychiatrist and reports from prison guards to determine whether a prisoner poses a risk to the public. In November, a three-person panel decided to release the 60-year-old under "stringent" licence conditions lasting at least 10 years. The details of such sessions are not available to the public. But it is known that Worboys must report to a probation officer every week, and is banned from contacting his victims.

"Given the nature of John Worboys' crimes, it is safe to assume that this was not a decision the Parole Board took lightly," Sam Elliott, criminal barrister at One Pump Court Chambers, told IBTimes UK.

Part of the uproar lies in the fact that Worboys was reported to have been handed a life sentence and attacked 100 women but only served a decade.

"Whilst still serious, this [19 charges of drugging and sexual assault] does not project his sentence into the echelons of decades in prison, as the press and public might like to see," Rachel Quickenden, senior associate at Corker Binning, told IBTimes UK.

"Worboys' sentence is in accordance with the law concerning the offences for which he was convicted. He was not convicted of hundreds of rapes and so cannot be sentenced on that basis."

In addition, Worboys wasn't handed a life sentence but given an Imprisonment for Public Protection (IPP) sentence, with a minimum tariff of eight years.

IPPs were introduced by the then home-secretary in 2003, and were created to jail serious offenders who could be given a life sentence for various reasons. They were scrapped in 2012, because they were deemed to violate human rights as thousands of prisoners were kept in prison beyond their minimum tariffs.

"An IPP sentence is different to a life sentence – it essentially means that a convict has to serve a minimum of eight years in prison. It is important to appreciate that this minimum period is set by the court, not the Parole Board. Only after these eight years would Worboys have become eligible to apply to the Parole Board for release," explained Elliott.

"It can be very difficult for a prisoner to persuade a Parole Board that they are suitable for release," he added. "It is likely therefore that Mr Worboys had powerful supporting evidence in his favour."

Barrister and author The Secret Barrister told IBTimes UK that whether the original sentence was too harsh or too lenient is a matter of debate, "although it is notable that the prosecution did not seek to 'appeal' it by referring it to the Court of Appeal as 'unduly lenient', which might suggest that they thought the sentence was appropriate."

Although it is impossible to comment on a case without knowing the full details, The Secret Barrister said he believes that Worboys would have served around another two and a half years in prison if he were convicted today.

"He would instead receive either a life sentence, which operates in a similar way [to the IPP], or more likely an extended determinate sentence (EDS), which can be imposed on dangerous offenders. The judge would fix a custodial term, say 16 years, and the defendant would be eligible for release at the two-thirds stage of the sentence -not the halfway point, as applies to standard sentences and was used to calculate the 8 year tariff for Worboys' IPP."

Given the uproar, however, will Worboys' case change how rape is treated by the legal system?

But as human rights barrister Adam Wagner explained on Twitter, it already has. Two of Worboys' victims won a landmark human rights case against the Metropolitan Police, accusing the police of systematically failing to investigate their complaints against Worboys. The subsequent Human Rights Act case arising from the criminal investigation should improve the treatment of alleged sexual assault victims in the future, he believes.

"There were multiple errors in the way that the police handled complaints in this case which will hopefully lead to changes to avoid a repeat of the mistakes," said The Secret Barrister.

There is, however, no reason why a Parole Board's decision on a specific case will affect how rape cases are dealt with in court, commented Elliot, adding: "The one thing that may change because of today's announcement and subsequent outcry is that the Parole Board decision making process may become more transparent in the future."

This piece previously state that defendants are eligible for release two or three years into their sentence. It has since been corrected to state that prisoners are eligible for release at the two-thirds stage of their sentence.