The flare, consisting of electrons and charged ions of elements from hydrogen to iron, was thrown off the Sun's surface in "a high energy event" known as a coronal mass ejection.
"When it gets to the Earth, the first effects will be seen by spacecraft and satellites," said Andrew Coates, of the Mullard Space Science Laboratories.
The charged particles induce currents in metal objects. The last time this occurred, in January, the Telstar 401 satellite failed. Power lines also suffer from the induced currents, which can blow supplies. In 1991, a Quebec power company was seriously affected by power surges caused by a solar storm.
Scientists were unsure yesterday afternoon exactly when the particles from the ejection will arrive in the Earth's magnetic field. It was detected by the Solar and Heliospheric Observatory, a satellite that Nasa and the European Space Agency launched in 1995 to keep a constant eye on the Sun's surface.
Dr Coates said: "We'll get about an hour's warning from one of our observation satellites.The Earth's magnetic field is going to take a battering. In January, the number of electrons in the atmosphere went up by a factor of 10,000. It was just after the peak that Telstar failed."
However, the same particles will also make the aurora borealis visible further south than normal. "It should be visible in Scotland, and maybe even lower down."
The real danger is to satellites which are in geostationary orbit - predominantly TV and communications. "They are in the firing line," said Dr Coates.