The insect is about the size of a grain of rice, say experts
Experts have been baffled by the presence of an unidentified insect in parts of London.
The tiny red and black bug first appeared in the Natural History Museum's Wildlife Garden in March 2007.
Since then it has become the most common insect in the garden and has also been spotted in Regent's Park and Gray's Inn.
The bug appears to be harmless, but there is potential for it to spread throughout the UK, said experts.
"It's not unusual to find something in the middle of a tropical rainforest," said Max Barclay, from the museum.
"But when you're eating your sandwiches on the lawn you don't expect to find something that takes you by surprise."
The insect was spotted on the seeds of some of the plane trees that grow in the museum's grounds.
The new insect is proving a mystery
Despite containing more than 28 million insect specimens, the museum failed to find an exact match for the new bug.
Experts said it closely resembles the rare species Arocatus roeselii that is usually found in central Europe.
But the roeselii bugs are brighter red than this new bug and they are usually associated with alder trees.
The National Museum in Prague discovered an exact match to the mystery insect but experts there have also failed to determine exactly what it is.
"It seems strange that so many of these bugs should suddenly appear," said Mr Barclay.
"There are two possible explanations - that the bug is roeselii and by switching to feed on the plane trees it could suddenly become more abundant, successful and invasive.
"The other possibility is that the insect in our grounds may not be roeselii at all."
The museum will continue to work with international colleagues to analyse the bug's body shape, form and DNA, to see whether it is a newly discovered species.