The purple flowers of the tiny green-winged orchid were first spotted by council staff maintaining the green roof of the Household Reuse and Recycling Centre in Holloway. Distinguished by the green veins on the outer parts of the flower - the ‘wings' that protect the petals - the rare, lone orchid requires highly specific conditions to germinate, and could be the first of many to appear on the site.

News of the find sent a buzz through the worlds of amateur and professional botanists and environmentalists as the orchid survives in only one or two other places in outer London - and nowhere in inner London.

Cllr Claudia Webbe, Islington Council's executive member for environment and transport, said: "This is a very exciting find and its importance should not be underestimated. It just shows that even in inner city, urban areas you can create the kind of conditions that encourage and nurture the rarest wildlife there is.I look forward to seeing what else will show up on the green roofs of our buildings in future!"

The orchid has been verified by Mike Waller, orchid specialist and London Wildlife Trust conservation ecologist, who said he was "amazed" when he heard of the discovery. It is thought the tiny dust-like seeds of the flower, which has seen its native hay meadow habitat decimated over the last 100 years, were blown by the wind onto the green roof, which was installed 12 years ago. They can take several years to flower after germinating.

Mr Waller, who has spent 15 years studying orchids, added: "There are only really one or two sites in London where it still flowers today. One of the most famous sites is Morden Cemetery, but it has not flowered there for about 10 years now. This is a great example of the quality and good management of this green roof."

Cllr Webbe made a special visit to see the rare specimen with Mr Waller and Dusty Gedge, president of the European Federation of Green Roof Associations and leading authority on green roofs. Mr Gedge said: "It's only a little thing but it's quite an exciting story. What's really fascinating for me is that you do something 12 years ago through the planning process - which has benefits for the wider area like holding storm water, cooling the building, improving air quality and visual appearance - and suddenly something as special as this turns up. And who knows? In the future there might be 200 of these here. This would not have happened without good planning."

Islington Council encourages green roofs on new developments and has approved almost 54,000sq m of them since 2006.

Green-winged orchid facts:

The green-winged orchid was first recorded in London by Charles Darwin

Its seeds are miniscule and can easily get blown into the upper atmosphere and be transported for many miles

The green-winged orchid has seen a huge decline in number, primarily because 99% of its favoured hay meadow habitat has been lost in this country in the last 100 years - the majority in the last 50 years due to agricultural intensification.

The orchid requires exacting conditions to germinate. Every orchid requires a particular fungus to be present in the soil in order to start life - adding it later will not work.

In this case, mycorrhizal fungus was already in the green roof's soil when the seed landed, triggering the germination process.

It can take five years or more between germinating and putting up a flowering spike.

The orchid's Latin name isAnacamptis morio.

Source: Islington Council, London