Messages found on devices linked to a 14-year-old boy from Blackburn led to a major anti-terror operation in Australia, UK police have said.
Greater Manchester Police said the boy was held on suspicion of involvement in preparing an act of terror.
Officers said they had uncovered a "credible terrorist threat".
Australian police confirmed it was linked to the arrest of five teenagers in Melbourne over alleged plans to target Anzac Day commemorations.
In a joint statement, Victoria Police and the Australian Federal Police said they "can confirm a link between the arrest of a 14-year-old boy in Blackburn, Lancashire, in the United Kingdom on Saturday with Operation Rising, a Joint Counter Terrorism Team operation."
Australian Police arrested five men on Saturday and two remain in custody. The other three have been released but police have said that one man is expected to be charged on weapons offences.
'No UK threat'
The 14-year-old was first arrested in Blackburn on 2 April.
He was detained again on Saturday after officers from the North West Counter Terrorism Unit (NWCTU) and Lancashire Constabulary searched an address in Blackburn under warrant and examined a number of electronic devices.
They allegedly discovered messages to a man in Australia.
Det Ch Supt Tony Mole, from the NWCTU, said: "We seized some media devices and we found some communications on those devices that were a concern to us.
"We thought it indicated a potential credible terrorist attack in Australia. We immediately contacted the Australian authorities and we've been working with them since in relation to the material that we uncovered."
He reassured the public they had not uncovered an "imminent attack in the UK".
Det Ch Supt Mole said a team of UK officers had now arrived in Australia to help with the joint investigation.
The teenager, who is being held on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism, remains in police custody.
It emerged over the weekend that British police are reviewing security operations ahead of the centenary of the Anzac landings following the foiled alleged terror plot in Australia.
Anzac Day commemorates the Australian and New Zealand Army Corps' World War One battle in Gallipoli.
Australians and New Zealanders in the UK traditionally observe the occasion, which takes place on 25 April each year, by holding services in Westminster and Hyde Park in central London.