A rare Amazonian cactus called the Moonflower has bloomed for what botanists believe is the first time in the UK.

Experts at Cambridge University’s Botanic Garden kept a night watch throughout the week so that they did not miss the flowering of Selenicereus wittii – an event which usually begins at sunset and is over by sunrise.

The Moonflower actually bloomed earlier on Saturday afternoon at around 3pm, and the garden said it was 28cm long.

The plant emits a sweet-smelling fragrance as it begins to bloom, with the same floral notes as a perfume endorsed by the pop star Rihanna, according to one nature writer.

But just two hours after it has fully opened its scent changes to a rancid smell, before closing up for good at sunrise.

A post on the garden’s Instagram said: “Oh it’s been worth the wait, what a beauty! Our Moonflower has opened & the smell is divine – similar to gardenia & jasmine atm & it’ll change (we think) into rotten veg smelling later.”

The unusual cactus spirals around tree trunks with leaf-like, flattened stems or pads and is found exclusively above the high waterline of the floodplain rainforests of the Amazon Basin.

Its white, nocturnal flowers usually reach around 27cm in length and their scent attracts their pollinators – two species of hawkmoth.

They are the only ones with long enough tongues to reach the nectar, which is deep within the plant.

Amazonian cactus
Glasshouse supervisor Alex Summers rare Amazonian cactus called the Moonflower (Joe Giddens/PA)

Alex Summers, glasshouse supervisor at Cambridge University’s Botanic Garden, said he was “excited” to share the blooming of the cactus, which was streamed live online.

“It’s very rare to have this plant in our collection and we believe this is the first time the Moonflower has flowered in the UK,” he said.

“I noticed the flattened stems, or pads, which swirl around the trunk of our Water Chestnut had sent out a flowerbud in late November – which was a lucky spot as it’s almost 12 feet up in the air and could have so easily been missed.”

He said it only recently “increased radically in size”, indicating that flowering was imminent.

He added that he loved the story of how British botanical artist Margaret Mee brought the plant to the attention of a wider audience.

She first saw the Moonflower in 1964 but had to wait until 1988 to see one in flower.

Her painting of it is held by Royal Botanic Gardens, Kew in south-west London.

Mr Summers said the cactus is expected to flower most years from now on.