A British teenager who pleaded guilty to inciting a terrorist attack at this year's Anzac Day parade in Melbourne will serve at least five years in jail.

The boy was only 14 when he masterminded the plot, to carry out a suicide attack on police officers at the parade, from his bedroom in northern England.

Relatives of the boy wept as judge Justice Saunders said that after careful consideration he had decided to impose a life sentence with a minimum term of five years.

The judge said the boy, now 15, had played a vital part in the plot by encouraging the intended attacker, supporting him and making sure he did not have second thoughts.

"Had the authorities not intervened, [the defendant] would have continued to play his part, hoping and intending that the outcome would be the deaths of a number of people," the judge told Manchester Crown Court.

The judge said it was chilling that someone so young could have become so radicalised that he was prepared to carry out this role, intending and wishing that people should die.

The boy, who cannot be named for legal reasons, will spend the first three years of his sentence at a youth custody facility.

The boy was arrested in March over threats to behead teachers, shortly before Australian police arrested five men in counter-terrorism raids in Melbourne.

In July, Detective Chief Superintendent Tony Mole said the boy was communicating with a man in Australia, adding that there was a "credible terrorist threat".

A security official with knowledge of the police operation told the ABC's 7.30 program in April that the plot involved running a police officer over and killing him with a knife.

Europe correspondent Barbara Miller describes the scene inside the court Nothing about this court is normal. The defendant who has pleaded guilty to inciting a man in Melbourne to run down and behead Australian police officers sits calmly in the bench behind the lawyers. He is flanked by his parents. He wears no handcuffs, because this is not a man but a boy, 15 now, but just 14 when he carried out his crime. He is in appearance completely unremarkable, smartly dressed in a grey shirt and a tie. He has opened the top button and loosened the tie a little. It looks almost like the school uniform a boy his age would be wearing if he wasn't facing sentencing for this horrific act. The High Court judge has come up from London to pass sentence so that the case can be heard closer to where the boy is being held in a youth custody facility, closer to his family. The judge has decided not to wear his wig and robes to make the court less intimidating. A son, a schoolboy, a hero on social media to supporters of the Islamic State group and their ilk, a convicted terrorist. After the sentence is passed the boy is allowed to hug his family, some of whom are weeping. He is then allowed to walk unescorted to a court door, where officers meet him to take him back into custody. It will be at least five years until he is seen in public again, it could be many more.

Those behind the plot then planned to take the officer's gun and go on a shooting rampage that would have ended in their deaths, the official told 7.30.

During a sentencing hearing prosecutors alleged that Australian Abu Khaled al-Cambodi was crucial to a terrorism plot by putting the boy in touch with a then-18-year-old Melbourne man, alleged to be Sevdet Besim.

Mr al-Cambodi was formerly known as Neil Prakash before he headed for Syria to join the jihadist cause.

It was alleged the British boy came into contact with Mr al-Cambodi on social media. Mr al-Cambodi had become a prolific user of social media, gathering thousands of followers within weeks, in what was described as the Jihadi Twitter community.

When the Melbourne man contacted the British boy, he began by saying: "I am the brother from Australia."

An intense nine days of communications between the pair followed, while they exchanged thousands of encrypted messages via a smartphone app, plotting the suicide attack on police officers during the Anzac Day parade.

Communications recovered from the teenager's phone showed them honing in on Anzac Day.

"Sounds good", the teenager wrote when the Melbourne man suggested it.

He responded: "Make the dogs remember this as well as there (sic) fallen 'heroes'."

The defence had argued the boy had made significant changes since he was placed in youth custody, and had shown his remorse through his guilty plea.

They argued it was the Melbourne man, and not the teenager, who was the driving force behind the plot, stopped only when British and Australian police arrested the men in a coordinated operation.

The Melbourne man currently faces terrorism charges in Australia.