It is the first time the plant - also known as "Queen of the Orchids" - has flowered since it was collected 32 years ago from Sabah, Malaysia.
Right now the plant stands 1.5m high but the flower spike is growing around 1.5cm a day and is expected to hit an astonishing three metres. The yellow blooms with delicate chestnut markings are expected to last from five to six weeks.
Kew tropical nursery supervisor Christopher Ryan said staff were surprised and thrilled when the plant produced a flower spike.
"It is incredibly rare for these orchids to flower in cultivation in Europe. We are so lucky at Kew to have a glasshouse large enough to grow such a big specimen and also to have the incredible expertise to give this orchid the amount of care and attention it needs to bloom. What an honour to be able to finally see it flower at Kew."
Grammatophyllum speciosum grows as an epiphyte in Indochina, Malaysia, Borneo and Sumatra, and in the wild only flowers every two to four years, and far less in cultivation. Rarely cultivated due to its immense size, a mature specimens leafy stems can grow several metres long. A plant weighing two tonnes was one of the highlights in the 1851 exhibition at Crystal Palace in London.
Visitors will be able to go behind the scenes of the Tropical Nursery to view this giant orchid. Free tours to see the plant will be available Wednesdays at 1pm and 2pm, starting at Whitepeaks cafe. Tours have limited space available and pre-booking is essential -email email@example.com to reserve a spot.