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Parry predicts that as flood or starvation strikes, the most dangerous zones will be Africa, particularly the northern half; most of the Middle East and central Asia as far as northern China; a strip from Nepal to Indonesia; and perhaps eastern China.

He pinpoints 2012 to 2018 as the time when the current global power structure is likely to crumble. Rising nations such as China, India, Brazil and Iran will challenge America’s sole superpower status.

This will come as "irregular activity" such as terrorism, organised crime and "white companies" of mercenaries burgeon in lawless areas.

The effects will be magnified as borders become more porous and some areas sink beyond effective government control.

Parry expects the world population to grow to about 8.4 billion in 2035, compared with 6.4 billion today. By then some 68% of the population will be urban, with some giant metropolises becoming ungovernable. He warns that Mexico City could be an example.

In an effort to control population growth, some countries may be tempted to copy China’s "one child" policy. This, with the widespread preference for male children, could lead to a ratio of boys to girls of as much as 150 to 100 in some countries. This will produce dangerous surpluses of young men with few economic prospects and no female company.

"When you combine the lower prospects for communal life with macho youth and economic deprivation you tend to get trouble, typified by gangs and organised criminal activity," said Parry. "When one thinks of 20,000 so-called jihadists currently fly-papered in Iraq, one shudders to think where they might go next."

The competition for resources, Parry argues, may lead to a return to "industrial warfare" as countries with large and growing male populations mobilise armies, even including cavalry, while acquiring high-technology weaponry from the West.

The subsequent mass population movements, Parry argues, could lead to the "Rome scenario". The western Roman empire collapsed in the 4th and 5th centuries as groups such as Ostrogoths, Visigoths, Suevi, Huns and Vandals surged over its borders. The process culminated in the sack of Rome in 455 by Geiseric the Lame, king of the Alans and Vandals, in an invasion from north Africa.

Parry estimated at the conference there were already more than 70 diasporas in Britain.

In the future, he believes, large groups that become established in Britain and Europe after mass migration may develop "communities of interest" with unstable or anti-western regions.

Any technological advantage developed to deal with the threats was unlikely to last. "I don’t think we can win in cyberspace — it’s like the weather — but we need to have a raincoat and an umbrella to deal with the effects," said Parry.

Some of the consequences would be beyond human imagination to tackle. The examples he gave, tongue-in-cheek, include: "No wind on land and sea; third of population dies instantly; perpetual darkness; sores; Euphrates dries up ‘to clear way for kings from the east’; earth’s core opens."

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Rear Admiral Chris Parry is the armed forces’ chief “blue skies” thinker.

Parry, 52, was educated at the independent Portsmouth grammar school and at Jesus College, Oxford. During the Falklands war in 1982, he was mentioned in dispatches while serving with the Fleet Air Arm on the destroyer HMS Antrim.

Parry is one of Britain’s leading specialists on amphibious warfare. He once commanded the assault ship HMS Fearless, was in charge of amphibious warfare training at Portsmouth naval base and headed a joint British-Dutch taskforce before moving to his post at the Ministry of Defence.

The admiral heads the development, concepts and doctrine centre, set up in 1998 and based at Shrivenham, Wiltshire. It has more than 50 staff and is being expanded to include extra analysts.

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