Italian police have arrested the fourth man wanted in the UK over the failed attacks in London on 21 July.
Osman Hussain was staying at his brother's flat in Rome
Osman Hussain - a Somali-born British citizen - was staying at his brother's flat in Rome when he was detained.
He is now being held at central police headquarters in Rome and is due to be interrogated by the two anti-terror investigators in charge of the case.
Britain wants him extradited as soon as possible, but it is not clear whether Italy might want to charge him there.
This is due to the fact that an Italian citizen, a woman, was among the victims of the 7 July bombings on the London Underground.
No link has yet been established between the 7 July attacks and the 21 July incidents.
The 27-year-old reportedly offered no resistance when special police forces broke down the door of the house in a Rome suburb where he was hiding.
Police are also said to have seized computers and CD-Roms from a call centre near Rome's Termini railway station run by the suspect's brother, who has also been taken into custody.
Mr Hussain was reportedly tracked down through mobile phone intercepts after British police gave their Italian counterparts a number he had been communicating with.
Armed police in London have also made a number of arrests, including a man wanted for the failed attack on a bus.
The arrest was "truly worthy of praise," said Italy's interior minister in a statement.
"The anti-terror operation, still ongoing, is part of wider international collaboration," Giuseppe Pisanu said.
"It confirms, not only the strength of our security systems, but also the efficiency of international co-operation."
Based on unnamed police sources, Italian state broadcaster Rai said he had left London two days ago - travelling to Rome via Paris and Milan.
Italians are worried by the fact that the suspect's presence in Rome might indicate that there are terror cells there as well, says the BBC's David Willey.