Britain's critical national infrastructure could be crippled in a high-altitude space attack by a rogue state or terrorists, MPs warned today.
A nuclear device detonated up to 500 miles above the earth's surface could generate an electro-magnetic pulse (EMP) with a "devastating" effect on power supplies, telecommunications and other vital systems, the Commons Defence Committee said.
It warned that countries such as Iran - which is resisting international pressure to end its nuclear programme - and even eventually some "non-state actors" could acquire the technology to mount such an attack.
Terrorists could also build a "crude" non-nuclear EMP weapon, with the power to cause disruption over a more limited area.
James Arbuthnot, the chairman of the committee, said he believed the possibility of an EMP attack was "quite likely".
"I personally believe that it's quite likely to happen. It's a comparatively easy way of using a small number of nuclear weapons to cause devastating damage," he told BBC Radio 4's Today programme.
"The consequences if it did happen would be so devastating that we really ought to start protecting against it now, and our vulnerabilities are huge."
Arbuthnot said that exploding a nuclear weapon in space in order to knock out electronic systems would actually be worse than if one was exploded on the ground in a city.
"The reason for that is it would, over a much wider area, take out things like the National Grid, on which we all rely for almost everything, take out the water system, the sewage system," he said.
"And rapidly it would become very difficult to live in cities. I mean within a matter of a couple of days.
But despite the vulnerability of the UK to such an attack, the committee accused the Ministry of Defence of appearing "complacent" and "unwilling to take these threats seriously".
It said ministers should start work on "hardening" the infrastructure to protect against an EMP attack "as a matter of urgency".
"It is time that the government began to approach this matter with the seriousness it deserves," it said.
The committee said the government currently rated the probability of a high-altitude EMP attack as "low", although it acknowledged that the impact would be severe.
However, an official EMP commission in the United States found "rogue states" such as Iran and nuclear-armed North Korea were well aware of the potential for such an attack.
The Iranians in particular were reported to have conducted missile tests which appeared to simulate the effects of an EMP nuclear strike.
The Americans concluded that in the event of such an attack, the widespread collapse of the electrical power system was "virtually inevitable".
"Certain states such as Iran could potentially pose a realistic threat in the future, even if it does not currently do so, if nuclear non-proliferation efforts are not successful. Non-state actors could also pose a threat," the committee said.
"While the risk may at present be low, the potential impact of such a weapon could be devastating and long-lasting for UK infrastructure. The government cannot therefore be complacent about this threat and must keep its assessment of the risk under review.
"It is therefore vitally important that the work of hardening UK infrastructure is begun now and carried out as a matter of urgency."
It is not only EMP weapons that have the power to wreak havoc. The committee said a naturally occurring "space weather event" caused by changing conditions in the sun's atmosphere could have a similar effect.
The most severe example - known as the Carrington event after the astronomer who observed it - occurred in 1859 when a massive solar flare sent enormous electrical currents surging through telegraph systems causing shocks to telegraph operators and setting fire to papers.
In 1989 the entire power grid in the Canadian province of Quebec collapsed in a matter of just 90 seconds after stabilising equipment failed to cope with the effects of a geomagnetic storm.
The likelihood a severe space weather event occurring over the next five years is currently assessed by the government as being "moderate to high", the committee said.
The National Grid has estimated that if there was another Carrington-sized event, there was a 91% chance that an area of the UK would be left without power for two months or more while essential satellite systems could also be damaged
The committee said it was now vital the government ensured back-up procedures and equipment were in place to meet the "reasonable worst-case scenario" for a severe space weather event.
It said ministers should also consider the practicability and cost of establishing resilience against a widespread loss of transformers, as could occur in an EMP attack.
"The potential threats of a Carrington-size space weather event or a high-altitude nuclear EMP weapon would have specific and potentially devastating impacts upon the electrical grid and other aspects of electronic infrastructure, which play an absolutely critical role in UK society," it said.
"It is therefore vital that the electrical grid is resilient as possible to potential threats such as these."
A government spokesman said: "We take these threats seriously, and proportionately, and are considering the Defence Committee report carefully. We shall respond fully in due course.
"Many of the points it raises are already coordinated across government and will be covered by the National Space Security Policy expected later this year."