Greater Manchester Police arrested the boy in Blackburn, Lancashire on April 2 under suspicion of preparing an act of terrorism, after officers examined a number of his electronic devices.

Detective Chief Superintendent Tony Mole said the UK counter-terrorism unit found evidence of communication between the boy and an Australian man of "what we believe is a credible terrorist threat".

"The minute we found it we were onto the Australians," Superintendent Mole told a press conference on Monday. He confirmed it was the information the UK shared with Australian authorities that put police on the path to foil the alleged Anzac Day plot.

The British 14-year-old was released on bail until May 28 but arrested again on Saturday, April 18, the same day five young Australian men were arrested in raids in Narre Warren, Eumemmerring and Hampton Park.

Three of the five Australians arrested in Saturday's anti-terrorism raids have been charged over an alleged Anzac Day terror plot. Two have been charged with conspiracy to commit terrorist acts, while another has been charged on summons with weapons offences.

"Investigators received information from UK authorities after they allegedly uncovered communications between the teenager and a man in Australia," Victoria Police said in a statement.

"The nature of the communication will form part of the investigation and therefore it would be inappropriate to comment further."

The 14-year-old Briton was arrested on suspicion of "being concerned in the commission, preparation or instigation of acts of terrorism" and remains in custody, the Greater Manchester Police said in a statement.

Superintendent Mole said he believed the boy was the youngest person ever arrested in Britain on suspicion of such a crime, the Guardian reported.

He said his officers had found no evidence of an imminent attack in the UK.