The young man who featured in the hit Netflix series Making a Murderer has secured a crucial legal victory after his conviction was overturned. If prosecutors do not appeal the decision, he will be freed from prison within 90 days.

Brendan Dassey, 26, a vulnerable young man, had been jailed for 41 years after being convicted of the murder of photographer Teresa Halbach. He and his uncle, Steven Avery, who was also convicted of the killing, became the subject of public outrage after the documentary suggested they had been framed, with the police coercing confessions out of Mr Dassey and allegedly planting evidence to snare Mr Avery. The show also highlighted how Mr Dassey's lawyer failed to properly defend his client.

Mr Dassey, who was aged seventeen-and-a-half at the time of his 2007 conviction for the 2005 killing, was told at his trial that he could not apply for parole until 2048.

The ruling says that prosecutors have 90 days to appeal the decision

But a federal judge in Milwaukee on Friday overturned the conviction, and also criticised the lawyer who was portrayed in the documentary as someone who had performed badly, and even colluded with prosecutors.

According to the judge, Mr Dassey’s confession was obtained by investigators who gave the teenager “false promises”. The rulling said that “investigators repeatedly claimed to already know what happened on October 31 and assured Dassey that he had nothing to worry about”.

The judge said that this, along with “Dassey’s age, intellectual deficits, and the absence of a supportive adult”, meant the confession was “involuntary”.

Brendan Dassey's confession was a crucial part of the case against his uncle (YouTube)

“Especially when the investigators’ promises, assurances, and threats of negative consequences are assessed in conjunction with Dassey’s age, intellectual deficits, lack of experience in dealing with the police, the absence of a parent, and other relevant personal characteristics, the free will of a reasonable person in Dassey’s position would have been overborne,” said federal judge William Duffin.

A spokeswoman for the state Department of Justice, which was handling the case, did not immediately return a message seeking comment, according to the Associated Press. The federal ruling comes after Mr Dassey’s appeal was rejected by state courts. The case's original prosecutor, Ken Kratz, who had defended the case after the series was broadcast, could not be contacted.

Making A Murderer- Where are they now?

At the teenager’s trial in Dane County, Wisconsin, the jury deliberated for four hours, finding Dassey guilty of first-degree intentional homicide, second-degree sexual assault, and mutilation of a corpse.

The 10-part series followed Avery’s 1985 wrongful conviction for rape and attempted murder, which landed him in prison for 18 years until he was exonerated. After his release in 2003, he sued the local authorities for $30m in damages.

It was at that point that he was accused of murdering Ms Halbach, a photographer who had photographed his car for Auto Trader magazine.

The series covered the controversy surrounding both arrests and subsequent convictions.

Steven Avery is one of two convicted murderers featured in the Netflix documentary (AP)

Friday’s ruling said the state has 90 days to appeal the federal order or else Mr Dassey must be released from prison. He is currently being held at Columbia Correctional Institution in Portage, Wisconsin.

Calumet County prosecutor Ken Kratz led the cases against Avery and Mr Dassey (AP)

Avery was tried and convicted separately in the killing. He is seeking a fresh trial and has secured the help of a leading defence lawyer, Kathleen Zellner, who is working with the Midwest Innocence Project.

On Friday, Ms Zellner said in a statement that Avery was thrilled to hear of the ruling for his nephew. “We know when an unbiased court reviews all of the new evidence we have, Steven will have his conviction overturned as well,” Ms Zellner said.

Earlier this year, she said on Twitter she had obtained a new DNA sample that she believed was crucial to Avery's case.