He conceded that he encouraged an 18-year-old Australian, Sevdet Besim, to murder police officers on Anzac Day in April, an annual celebration commemorating the Australian and New Zealand Army Corps’ World War I battle in Gallipoli.

The boy, who was 14 at the time he was plotting with another and is the youngest person in Britain to be charged with terror offences, entered his surprise plea via video link from Manchester Crown Court.

Wearing a grey shirt and tie, the boy spoke only to plead guilty.

The teenager is now being held at a youth detention centre, and sentencing has been adjourned until 3 September.

“The messages revealed the intention of the plot and the target, along with motivation summarised as support for ISIS and enthusiasm for the attack”.

That exchange was the subject of a second charge, of inciting terrorism overseas in relation to beheading a person in Australia, which has now been dropped by the prosecution.

Judge John Saunders ordered pre-sentence reports in particular to find out if there was any “indoctrination”.

His arrest came after Australian police arrested five men in counter-terrorism raids in Melbourne on April 18.

Mr Greaney told the court this plot would be “part and parcel” of the allegations that the boy has now admitted.

In one message, the teenager told Besim: “Suggest you break into someone’s house and get your first taste of beheading”.

They had allegedly planned to run down police officers with a vehicle, behead them and then shoot other people, according to Australian media reports.

“The messages also set out the plotters” preparations for the attack.

“Sevdet Besim said this seemed a little risky”.

Besim and Causevic were reportedly being monitored because of their contact with a man who was shot dead by police in Australia last year after stabbing two officers.