There have been sightings in the south Midlands, Greater Manchester, Scotland and Northern Ireland.
The plant grows very quickly and contains chemicals known as photo-sensitising furanocoumarins.
These substances prevent the body from protecting itself from UV light and can lead to severe burns.
A red rash develops on the skin, which grows more painful and then blisters.
Guy Barter of the Royal Horticultural Society said: "Wherever you live in the UK, you can expect to encounter this plant".
The facts: what does it look like
- Giant hogweed (Heracleum mantegazzianum) is a close relative to cow parsley.
- It has thick, bristly, reddish-purple stems and can reach over 3m (10ft) in height.
- The flowers are white in a flat-topped clusters that can be as large as 60cm (2ft) across.
- Giant hogweed was originally brought to Britain from Central Asia in 1893.
- It commonly grows on riverbanks and wasteland.
- Its leaves, stems, roots, flowers and seeds contain toxic components which can be transferred by contact and make exposed skin extremely sensitive to sunlight.
What to do if you come into contact
After coming into contact with the plant, the burns can last for several months and the skin can remain sensitive to light for many years.
NHS advises: “If the sap of the giant hogweed comes into contact with your skin, it can cause severe, painful burns and make your skin sensitive to strong sunlight.
“If you touch a giant hogweed, cover the affected area, and wash it with soap and water.
“The blisters heal very slowly and can develop into phytophotodermatitis, a type of skin rash which flares up in sunlight. If you feel unwell after contact with giant hogweed, speak to your doctor.”