Australian police, already holding one Indian doctor since Monday, have extended their probe to at least five others and searched two hospitals.
Brown said he had spoken to John Howard, the Australian prime minister, about the probe into the suspected al-Qaeda-linked plot, in which eight Middle Eastern and Indian medics have been arrested.
"This is an international investigation now," Brown said of the hunt for those behind the failed car bombs in London and an attack on Glasgow airport by two men who smashed a jeep into a terminal building.
"I believe that, from what I know, we're getting to the bottom of this cell that has been responsible for what is happening," he told the BBC.
Brown has spoken of al-Qaeda links to the latest plot, and British intelligence services are working with counterparts around the world to establish the extent of suspected involvement by Osama bin Laden's network or its Iraqi arm.
There is a high level of security in London this weekend, with the start of the Tour de France cycle race in London and the finals of Wimbledon.