Police fear an Isis-linked terror cell exploited migrant routes in to Europe to plot terror attacks on the UK as Britain's Home Secretary Theresa May ordered a shake-up of the country's coastal defences.
The group photographed suspected targets in London, including hotels, restaurants and a health centre and may have planned to slip attackers in through Calais.
The alleged plot was unearthed after two Afghan men and a Pakistani man were arrested in Italy.
They had been granted refugee status in the country meaning they would have been able to take advantage of Europe's open borders to move freely around the continent.
They are part of a suspected five-man cell, of which at least one was in London in December potentially scouting targets.
May, the Home Secretary, suggested that Britain is preparing to increase its "maritime capacity" amid fears migrants are increasingly trying to access the UK through small ports on the Kent coast.
There are currently four Border Force cutters patrolling Britain's coast. However, experts have warned that the fleet is too small to protect the country from people smugglers.
Keith Vaz, the Chairman of the Home Affairs select committee, asked May whether it is true that she has "authorised a large increase in the maritime capacity of the border force to deal with activities off the Kent coast".
The Home Secretary replied: "The maritime capability of the border force is being changed so that it is more flexible to be able to deal with the issues that we are now looking at and addressing because we are conscious of the need always to ensure that the capability is able to deal with any potential issues that may be coming down the line."
Downing Street is concerned that any escalation of the migrant crisis in the coming months could effect the result of the European Union referendum, amid claims from eurosceptics that Britain's borders are not sufficiently secure.
Separate security fears also emerged in France for the Cannes Film Festival and the forthcoming Euro 2016 football finals amid the growing terror threat.
Italian investigators suspect the latest group were planning attacks in Britain, Italy, France or Belgium.
They believe they might have also been a people trafficking gang and could have provided "logistical support" for a terror group linked to Isis (Islamic State).
One of the detained men, Gulistan Ahmadzai, 29, was suspected of planning to move to Calais, raising the prospect of using that as a base to either enter the UK or assist others.
Photographs of another of the arrested men, Hakim Nasiri, 23, showed him previously posing with an automatic rifle.
Images found on electronic devices allegedly showed apartments, a health centre, restaurants and a hotel at West India Quay, the Sunborn Yacht Hotel in Royal Victoria Dock, the South Quay footbridge to Canary Wharf, and an Ibis hotel in Victoria Dock Road, all in London.
Other alleged targets included a Premier Inn Hotel in International Square in Stratford in the east of the city, which is situated inside a Westfield shopping centre and around 200m from the Olympic Park.
There were also images of an Italian airport and a port, the Colosseum in Rome and potential targets in France.
Roberto Rossi, an Italian prosecutor, told a press conference: "It was clear these were not tourist images. They appear to have been scouting sensitive sites."
Giuseppe Drago, another public prosecutor, told Italian newspaper La Repubblica: "They planned to carry out terror attacks against government and military targets, public institutions, international companies and civilian targets.
"They had weapons as well as photo and video documentation which had been taken during preliminary surveys of potential targets."
Two other suspects, Afghans Qari Khesta Mir Ahmadzai and Surgul Ahmadzai, are still wanted by the police and may now be in Afghanistan.
Surgul Ahmadzai, 28, was in London on December 9 last year and then flew to Bari in Italy via Milan, police said.
Two days later he flew to Paris, returned to Italy on December 15 and then, along with Qari Khesta Mir Ahmadzai, 30, flew from Bari to Istanbul and from there to Kabul.
Rossi said members of the alleged cell travelled around Europe without having "the apparent financial means or legitimate reasons for doing so," suggesting that their journeys were paid for by contacts, possibly from a terror group.
Nasiri and Gulistan Ahmadzai were both arrested in the southern Italian city of Bari, where Nasiri was still staying in a refugee centre.
He had only been granted asylum last week while his co-accused was granted asylum in 2011.
A third man, Pakistani Zulfiqar Amjad, 24,was arrested in Milan but was also living in Bari.
Nasiri was arrested on suspicion of terrorism while Ahmadzai and Amjad were held on suspicion of aiding and abetting illegal immigration.
Centre-right politician Roberto Calderoli said the Italian government was "allowing in hundreds of thousands of illegal immigrants without the slightest controls, without knowing who they are, where they come from, how they paid for their journeys.
"Now we find that we have terrorists at home, welcomed into our migrant reception centres."
In a separate development security at the Cannes Film Festival is at an all-time high amid fears of a terror attack.
Organisers are reported to have brought in 400 extra members of its security team, with police and special forces on high alert following terror attacks in Paris last year.
There are also fears that a huge 'fan zone' to be set up near the Eiffel Tower during the Euro 2016 football Championship will "offer terrorists a chance for a massacre," according to a former French police chief.
Frédéric Péchenard, who headed the national police until 2012, urged the authorities to scrap plans for the Paris fan zone, where spectators will gather to watch matches on giant screens.