Insect experts at the Natural History Museum thought they'd seen it all - they do have 28 million specimens in their collection, spanning roughly half a million species.

That was until a tiny red and black bug, no bigger than a grain of rice, turned up in their own back garden.

Despite a year's efforts from specialists across Europe, the mystery insect has defied all attempts to identify it.

Max Barclay, an entomologist at the museum, came across the bug last spring. “I was in the gardens with my son and there was one under the gate,” he told The Times yesterday. "I thought, ‘That looks interesting, I've never seen anything like that before'.”

The bug was the most common insect in the museum's wildlife garden last summer and has since been found across southwest London, leading Mr Barclay to believe that it will soon spread across the country. But although the museum holds the world's largest collection of insects, no exact match could be found.

The bug closely resembles a Central European species, Arocatus roeselii, but it is a darker red and lives on plane trees rather than alder.

“It's a bit unsatisfactory that in the garden of the biggest museum in the world there was an insect that we couldn't identify,” Mr Barclay said.

Specimens have been found in Battersea Park, Chelsea Embankment and Gray's Inn in London, and Mr Barclay believes that the insect has now spread across the capital. “It seems to be on any streets and parks in London with plane trees,” he said.

It will not be long, he believes, before the bug makes the jump to other British cities where planes have been planted.

The bug has since been matched to unidentified specimens found in Nice and Paris, but Mr Barclay does not think that it is native to Europe.

“A native species would have predators and parasites that would keep its numbers under control. It could be from anywhere plane trees occur, which doesn't narrow it down very much.” Plane trees are found across the northern hemisphere from China to North America.

The bug lives off the seeds of the plane tree and is thought to be harmless to human beings and the trees.

It is only one of the many foreign invaders that are establishing themselves in Britain, Mr Barclay says. “With international trade and climate change, several new insects are showing up in London every year. Some of the invaders come from southern Europe, but others are from as far away as Australia. The fauna of the city is changing all the time now.”

Scientists in the Netherlands will now examine the creature's DNA in an attempt to find out more about its make-up.

Mr Barclay believes that this will rule out the possibility that it is a hybrid of a known species, and set researchers on the way to solving the mystery.

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