An asylum seeker among three arrested as part of an Italian counter-terror and human trafficking operation may have travelled around the UK scouting potential targets, reports suggest.
Hakim Nasiri, from Afghanistan, was detained with two other men after pictures of potential targets in London, Paris and Rome were found on mobiles.
An airport, port and shopping mall are feared to have been on a possible hit list drawn up by a group "making preparations", according to Italian prosecutors.
The 23-year-old posted pictures online of himself outside Buckingham Palace and at the Shard, the Daily Mail said, adding that he also claimed to be a student and the manager of a takeaway in Birmingham.
Nasiri was arrested at a home for asylum seekers in Bari. He is understood to have been held on suspicion of subversive association with the goal of international terrorism.
Two other men, Afghan Gulistan Ahmadzai, 29, and Pakistani Zulfiqar Amjad, 24, who were arrested in Milan on suspicion of aiding illegal immigration, had obtained international protection status in Italy.
Two more suspects are thought to have returned to Afghanistan.
The paper quoted Vincenzo Molinese, a colonel in the Bari military police, saying images found on Nasiri's phone of him holding a machine gun "were probably taken in the back room of a supermarket in England".
At a news conference on Tuesday, prosecutor Roberto Rossi said there was no evidence an attack was imminent, but added: "It is clear they were making preparations."
Among the images on the mobile phones were the airport, port and a shopping mall in Bari.
Mr Rossi acknowledged that the men's nationalities and the presence of pictures of sites around the city alone did not indicate sinister intentions.
But he said the prevalence of photos of sensitive sites "where tourists in general don't take pictures ... taken together, they assume an extremely strong meaning".