The roof of a City bank might not seem a natural spot for a rare orchid to thrive – but a plant thought to be extinct in the UK countryside has been found in this most improbable of locations, and no one is sure how it got there.

The small-flowered tongue orchid (Serapis parviflora) was discovered on the "green roof" of the Japanese investment bank Nomura during a regular plant survey earlier this month.

Mark Patterson, the ecologist who manages the roof, said seeds or baby plants could have been hidden in the soil used to create the garden a decade ago.

They can take many years to mature when grown in dry conditions and poor soil, and an alternative explanation is that the seeds were blown onto the roof, which is 11 floors above street level.

The dry, hot conditions found in the garden are thought to be ideal for the Mediterranean species, which share the roof with London’s largest colony of green-winged orchids, another of Britain’s rarest wild flowers.

Mr Patterson said: "Orchid seeds are incredibly small and can travel great distances by wind. The plants could have originated on the continent and been brought over the Channel on southerly winds which frequently bring Saharan dust deposits to the capital.

"Once settled on the Nomura roof, the seeds would have formed a symbiosis with a mycorrhizal fungus enabling them to germinate and grow – while possible, the odds are astronomical."

The skyscrapers of London form an unlikely backdrop to the site of a newly-discovered small-flowered tongue orchid

Credit: Nomura

The orchid is common in the Mediterranean basin and Atlantic coast of France, Spain and Portugal but very rare in the UK. The 15 plants are now thought to be the only wild examples in the country after a previous colony discovered in 1989 was destroyed in 2009.

Mike Waller, an orchid expert and the author of "Britain’s Orchids: A Field Guide to the Orchids of Great Britain and Ireland", said: "To find Britain’s second colony of small-flowered tongue orchids is exciting in itself, but to find them on a green roof in the City of London is extraordinary – on another level, if you’ll excuse the pun.

"This is clear evidence that, with patience and dedication, even the most unlikely places can become havens for some of our rarest wildlife."

Its unlikely location may also have been key to its survival. Rare plants in the wild are often dug up by unscrupulous collectors, and the location of the previous colony, in Cornwall, was kept secret to preserve it. It was eventually destroyed by land mismanagement.

But the location of this colony, on the roof of a private bank, means it is safe from disturbance.

Species native to Europe, including insects and plants, are expected to thrive in the UK as the climate changes to become warmer.

While this orchid is not native to the UK, it is not classed as invasive because it does not negatively impact existing flora and fauna.

Foreign species can arrive due to human influence, such as on imported plants and seeds, as well as being carried on the wind.