(CNN) Australia's High Court has agreed to consider an appeal against Cardinal George Pell's conviction on five counts of child sex abuse.

The decision handed down Wednesday in the Australian capital, Canberra, is the former Vatican treasurer's last chance to overturn his conviction and six-year sentence.

The appeal is expected to be heard by a panel of Australia's most senior judges sometime after March next year.

A former close adviser to Pope Francis, Pell was found guilty in a unanimous jury verdict last December of five charges of assaulting two 13-year-old choirboys in the mid-1990s.

He is the most senior Vatican official to be convicted of child sex abuse.

The first of two separate attacks for which he was convicted occurred after Sunday mass at St. Patrick's Cathedral in Melbourne, where Pell was archbishop and a revered figure within the Catholic Church. The second came a month later and involved a fleeting moment in a corridor when Pell grabbed the victim's genitals. Pell maintains he is innocent.

The 78-year-old cardinal has already spent several months in custody at the Melbourne Assessment Prison, as his legal team fights to overturn the conviction. He will remain in prison while his legal team compiles their High Court submission.

Pell's longtime spokeswoman Katrina Lee issued a statement saying, "this matter is now still before the court and so we are unable to comment."

Lower court appeal failed

Earlier this year, they took his case to the Victorian Court of Appeal, arguing in part that the jury couldn't have found Pell guilty beyond reasonable doubt based on the evidence. The evidence largely rested on the testimony of one of the two choirboys, now a man aged in his 30s. Neither man can be identified due to an Australian law protecting victims of child sex abuse.

In August, the appeals court rejected the submission of Pell's legal team by a 2-1 majority ruling.

In their ruling, Chief Justice Anne Ferguson and Justice Chris Maxwell found that the choirboy was "clearly not a liar, was not a fantasist and was a witness of truth." The dissenting judge, Justice Mark Weinberg, said discrepancies in the victim's account left him with some doubt. Weinberg's dissenting ruling was considered to have left an avenue open to take the case to the High Court.

In their submission, Pell's team is challenging the appeals court ruling, arguing the judges should have found the jury must have had a reasonable doubt about Pell's opportunity to commit the crime.

Former choirboy testified in court

Wednesday's High Court decision extends a four-year legal battle that started in 2015 when the former choirboy made a statement to Victoria Police.

In testimony to a closed court, the man said Pell had caught him and his friend drinking communion wine in the priest's sacristy after mass one Sunday. He said Pell parted his robes and forced them to engage in sex acts, one by one, despite their sobs and pleas for him to let them go.

The second choirboy did not tell anyone about the abuse and died from an accidental overdose several years ago after years of drug use. The surviving choirboy said he decided to come forward after attending his friend's funeral.

"Some commentators have suggested that I reported to the police somehow for my own personal gain. Nothing could be further from the truth," he said in a statement in August.

"I have risked my privacy, my health, my wellbeing, my family. I have not instructed any solicitor in relation to a claim for compensation. This is not about money and never has been.

"I felt I should say what I saw and what happened to me. I had experienced something terrible as a child, something that marked my life. I wanted at least some good to come of it," he said in the statement.

The Holy See previously said it would await the outcome of any Australian legal appeal before considering Pell's future within the clergy. It has not yet responded to Wednesday's High Court decision.

In a statement, the president of the Australian Catholic Bishops Conference, Archbishop Mark Coleridge, said it was Pell's right to seek a High Court ruling.

"This will prolong what has been a lengthy and difficult process, but we can only hope that the appeal will be heard as soon as reasonably possible and that the High Court's judgment will bring clarity and a resolution for all," he said.