The ghost orchid was officially declared extinct in Britain in 2005, with the last sighting recorded in Buckinghamshire in 1986.
Now the wildflower - a pale plant with no leaves - has been found in an oak wood in Herefordshire.
The sighting, made last autumn but initially kept secret, has been described as ''wonderful news'' by the botanical community.
Trevor Dines, of conservation charity Plant life, said: ''The rediscovery of a single ghost orchid is wonderful news - and gives us hope that this delicate species is not gone for ever.
''However, just as one swallow doesn't make a summer, one ghost orchid doesn't constitute a viable long-term population of a species, which has now been re-classified by the Vascular Plants Species Status Assessment Group as critically endangered.''
The orchid, which flowers between June and October, grows to 25cm (10in) tall and was traditionally found in deep leaf litter.
But numbers plummeted through a combination of its natural habitat being depleted and the use of herbicides and pesticides.
''Applying the extinct label is almost always difficult because plants can turn up later from seed or following better surveys of suitable sites - it's easy to tell if large mammals are extinct but not small plants that appear only sporadically,'' said Dr Dines.
''This is especially true of ghost orchid but plants declined at the last known site in Buckinghamshire from 25 plants in 1953 to five in 1986 - and it had not been seen at any of its previously-known sites since then.''
Plant life used the ghost orchid as the symbol of its conservation campaign, aimed at preventing the loss of wild flowers.
The manifesto was launched in September - the same month that the plant was rediscovered but before the find was made public.