The rarest species of orchid in Europe has been rediscovered on a volcanic range in the Azores, scientists report.

Although it was first documented in 1838, nearly two centuries have passed since the Hochstetter's butterfly-orchid was last seen, But botanists working on the Azores report their investigation has not only led to the rediscovery of the orchid, but that there are three different species of the rare flower.

Writing in the open-access peer-reviewed journal PeerJ, the botanists report that the butterfly-orchid is in urgent need of proactive conservation efforts.

The Azores are an excellent climate for this type of research, the scientists said, because the archipelago is located nearly 1,000 miles away from Portugal, the nearest land mass.

When their investigation began, they debated among themselves whether they would find one or two butterfly-orchid species. They easily found examples of short-spurred butterfly-orchid, which was widespread on all of the islands. While the narrow-lipped butterfly-orchid was rarer, it was only found on eight of the Azores islands.

But on top of a volcanic ridge, the team documented a third species.

Lead researcher Richard Bateman from the Royal Botanic Gardens Kew told BBC Nature that he was "astonished" to find this third species growing in such a landscape.

University of the Azores botanist Mónica Moura first spotted the rare third species.

"I immediately recognized the flowers as being exceptionally large for an Azorean butterfly-orchid and e-mailed images to Richard Bateman for confirmation that they were new to science," Moura said in a statement.

While the species seemed to be new, Bateman looked into the botanic record and found that the species has been documented almost 200 years ago.

"I was even more astonished when my subsequent studies in herbaria and libraries showed that this exceptionally rare orchid, found only on one mountain-top on a single Azorean island, had in fact been found by the very first serious botanist to visit the Azores, in 1838," Bateman told the BBC.

The "new" orchid has been illustrated but never correctly identified as a new species.

"This remarkable species languished unrecognized for 173 years," Bateman said in a news release. "It's rediscovery and recognition beautifully illustrate the value of integrating field-based and laboratory-based approaches to generate a modern monograph. This methodology both demonstrates that the species is genuine and allows us to make informed recommendations for its future conservation."