The highly-invasive species, which can cause severe blisters, burns and even blindness, has been reported in the south and east of England, the north-west near cities including Manchester, Liverpool and Leeds and in Scotland and Ireland.
Infestations are rife in Scotland and the north of England, according to the Environment Agency.
The so-called epidemic has become much worse in recent years as the plants have spread from their river bank habitats to roadsides, pathways and parks.
Hogweed is spreading across Britain
Infestations are rife in Scotland and the north of England, according to the Environment Agency
It comes as it emerged Scottish schoolboy Adam Hodgson, 11, was left in agony with horrific blisters after he came in to contact with the poisonous specimen in a park in Renfrewshire.
The Environment Agency has established a plant tracker app to pinpoint non-native species in the UK, meaning members of the public can record sightings of the dangerous Giant Hogweed wherever they find it.
The results show Hogweed can be found almost anywhere, but mainly on river banks and the toxic components in the leaves, stems, roots and flowers can be transferred to skin by touch.
Experts have warned the reaction can begin just 15 minutes after contact with the toxic plant, and heat and moisture can worsen the skin reaction.
Adam was left injured after coming into contact with giant hogweed Giant Hogweed: Britain's most dangerous plant Tue, July 21, 2015
A member of the carrot family, Giant Hogweed was introduced to the UK in the 19th century. In recent weeks, the toxic plant has been spreading across the country, leaving people with painful blisters, rashes and serious burns.
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Adam’s ordeal began when he was stung by nettles at the park.
He proceeded to rub what he believed was a dock leaf on the wound in the hope that it would take away the pain.
But the youngster actually used Giant Hogweed, which has recently been dubbed Britain's most dangerous plant.
The toxic sap was originally brought into the UK from Central Asia in 1983 as an ornamental plant and stayed in private gardens.
It is only in the last 20 years that is has spread to UK riverbanks and wasteland.
The NHS advises anyone who comes into contact with it to immediately wash the affected area with hot soapy water.
The toxic sap was originally brought into the UK from Central Asia in 1983
The resilient vegetation is “tall, cow parsley-like plant with bristly stems that are often purple-blotched” and flowering stems often reach a height of between two to four metres.
Each flower has 80,000 seeds, which means one plant can cause hundreds more to flourish.
Dispersed along long distances by water makes it a hazard for walkers, cyclists and gardeners up and down the country.
The annual cost of all invasive, non-native species to the UK economy amounts to £2 billion.
The toxic plant can leave people with blisters, burns and can even blind people
An Environment Agency spokesperson said: "Giant Hogweed can spread rapidly along watercourses forming dense colonies that suppress the growth of native plants and grasses.
"The public can find advice on the gov.uk website and report the spread of invasive plants through the plant tracker app."