The National Bee Unit confirmed the sighting at an apiary near Woolacombe, Devon, and work is under way to identify, destroy and remove any nests in the area.
Experts said the Asian species, which at up to 2.5cm (1in) long is smaller than native European hornets, poses no greater risk to human health than bees.
But it preys on important pollinating insects such as honeybees and could do serious damage to colonies here, which have not evolved to cope with the threat.
Asian hornets arrived in France in 2004 and are now common across large areas of Europe, with experts on standby in the UK in recent years for the insect's arrival here, from imports such as plants or timber, or even by flying across the Channel.
The species was discovered for the first time in Channel Islands Jersey and Alderney last summer.
The sighting in Devon is the first confirmed incident since last September when a nest was discovered in the Tetbury area of Gloucestershire. It was successfully contained by bee inspectors.
Work to contain the Devon case includes setting up a surveillance zone around north Devon, deploying bee inspectors who use infrared cameras and traps to track hornets and locate any nests, and readying disposal experts to use pesticides to kill the hornets and destroy the nests.
Nicola Spence, Environment Department (Defra) deputy director for plant and bee health, said: "While the Asian hornet poses no greater risk to human health than a bee, we recognise the damage they can cause to honeybee colonies.
"That's why we are taking swift and robust action to locate and destroy any nests in the Devon area following this confirmed sighting."
She added that the Government is remaining vigilant across the country and working with the National Bee Unit and its network of inspectors to protect bees from the Asian hornet threat.
Members of the public can download the free Asian Hornet Watch app to report possible sightings and send pictures of suspect insects to experts at the National Bee Unit..