Nuclear programmes in North Korea and Iran concern Western nations
Scientists have warned that Iran and North Korea could produce a weapon capable of paralysing Western electricity grids for months or years.
Experts fear that a missile-launched nuclear bomb exploded above the earth's atmosphere could cause a catastrophe.
They told the British government that high-altitude electromagnetic pulses could lead to an "economic shutdown".
US government adviser Avi Schnurr said efforts to protect the US electricity grid had already begun.
Mr Schnurr is president of the International EMP Council, a coalition based in the US, which aims to stimulate efforts to protect against the risk of attacks using electromagnetic devices.
Those feeding into the network include scientists from around the world, governments, corporations and NGOs.
On Monday, the British Parliamentary Defence Select Committee heard how a nuclear warhead launched from a medium range missile and exploded 20 miles above the earth could cripple an entire country's power grid.
HOW HEMP WORKS
The BBC's security correspondent Frank Gardner said this was called HEMP (High Altitude Electro Magnetic Pulse) and its effects could, in theory, bring a country to its knees.
At the briefing held in London, Mr Schnurr said Britain was still "just at the thinking and planning" stage and more action needed to be taken.
Mr Schnurr said that power, water and phones could fail.
Cars could run out of fuel, cities could be gridlocked and the economy could in effect be "shut down".
Recovery, he said, could take between four to eight years.
The scientists, who advise the US government, said the biggest concern for defence planners was the threat from so-called "rogue states" like Iran and North Korea.
The latter has already tested a nuclear weapon and fired a missile over the Pacific Ocean.
Iran denies seeking a nuclear bomb but critics say it has concealed much of its nuclear programme to date.