A huge mass of electrically-charged particles thrown out by a gigantic eruption on the Sun is due to strike the Earth tonight.
Scientists expect it to trigger one of the most violent geomagnetic storms ever recorded.
The result could be widespread power surges and even blackouts, disrupted TV and mobile phone signals, and broken down communication satellites.
At the same time the Northern Lights, normally confined to polar latitudes, may produce dazzling displays in the skies above southern Britain.
At least one satellite has already been knocked out of action by the storm. Japan's space agency said its Kodama communications satellite had been temporarily shut down after malfunctioning.
The solar flare that caused the eruption burst out of a sunspot at 10.54am yesterday.
Experts said it was the strongest flare seen in the past 30 years. The explosion caused a coronal mass ejection (CME) which is now speeding towards Earth.
Last week another CME only hit the Earth a glancing blow, yet was able to disrupt airline communications.
The solar flare was classified as an X18-category explosion, meaning it can trigger planet-wide radio blackouts and long-lasting radiation storms.
Although the charged particles present no direct danger to people on the ground, they could have a devastating effect on electrical equipment.
Geomagnetic storms are classified on a scale of one to five. Initial indications are that the looming storm could reach the highest G5 level and last for 24 hours.
The most obvious manifestation of the storm is likely to be glorious auroras lighting up the night sky.
Usually the Northern Lights, or aurora borealis, is only visible in Britain from northern Scotland. The Earth's magnetic field channels the solar particles that cause auroras towards the poles.
But experts say that over the next two days the Northern Lights may be seen at latitudes as low as Florida and Texas - and even south enough to cover the whole of Europe.