In an apocalyptic vision of security dangers, Rear Admiral Chris Parry said future migrations would be comparable to the Goths and Vandals while north African "barbary" pirates could be attacking yachts and beaches in the Mediterranean within 10 years.

Europe, including Britain, could be undermined by large immigrant groups with little allegiance to their host countries — a "reverse colonisation" as Parry described it. These groups would stay connected to their homelands by the internet and cheap flights. The idea of assimilation was becoming redundant, he said.

The warnings by Parry of what could threaten Britain over the next 30 years were delivered to senior officers and industry experts at a conference last week. Parry, head of the development, concepts and doctrine centre at the Ministry of Defence, is charged with identifying the greatest challenges that will frame national security policy in the future.

If a security breakdown occurred, he said, it was likely to be brought on by environmental destruction and a population boom, coupled with technology and radical Islam. The result for Britain and Europe, Parry warned, could be "like the 5th century Roman empire facing the Goths and the Vandals".

Parry pointed to the mass migration which disaster in the Third World could unleash. "The diaspora issue is one of my biggest current concerns," he said. "Globalisation makes assimilation seem redundant and old-fashioned . . . [the process] acts as a sort of reverse colonisation, where groups of people are self-contained, going back and forth between their countries, exploiting sophisticated networks and using instant communication on phones and the internet."

Third World instability would lick at the edges of the West as pirates attacked holidaymakers from fast boats. "At some time in the next 10 years it may not be safe to sail a yacht between Gibraltar and Malta," said the admiral.

Parry, 52, an Oxford graduate who was mentioned in dispatches in the Falklands war, is not claiming all the threats will come to fruition. He is warning, however, of what is likely to happen if dangers are not addressed by politicians.

Parry — who used the slogan "old dog, new tricks" when he commanded the assault ship HMS Fearless — foresees wholesale moves by the armed forces to robots, drones, nanotechnology, lasers, microwave weapons, space-based systems and even "customised" nuclear and neutron bombs.

Lord Boyce, the former chief of the defence staff, welcomed Parry’s analysis. "Bringing it together in this way shows we have some very serious challenges ahead," he said. "The real problem is getting them taken seriously at the top of the government."

Ancient Rome has been a subject of serious public discussion this year. Boris Johnson, the Conservative MP and journalist, produced a book and television series drawing parallels between the European Union and the Roman empire. Terry Jones, the former Monty Python star, meanwhile, has spoken up for the barbarians’ technological and social achievements in a television series and has written:

()"We actually owe far more to the so-called ‘barbarians’ than we do to the men in togas."

Parry, based in Shrivenham, Wiltshire, presented his vision at the Royal United Services Institute in central London. He identified the most dangerous flashpoints by overlaying maps showing the regions most threatened by factors such as agricultural decline, booming youth populations, water shortages, rising sea levels and radical Islam.

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