According to the Natural History Museum,, external reports of the bright green birds appearing in the countryside began to emerge in the late 19th Century.
It had been a popular cage-bird but there are currently estimated to be about 15,000 wild pairs, according to British Trust for Ornithology.
Carter said the birds have been spotted outside London as they moved across the country.
He said: “They are just expanding generally – they tend not to go very far from where they hatch and they are quite colonial.
“So they build and become bigger numbers where they are and then they start spreading out.
“It’s part of a slow spread out of London – they are well into Kent now and they are starting to get into the east now, so we are seeing a few in the west side of Essex.”
While speaking to BBC Essex’s breakfast presenter Sonia Watson, Carter said there have been some urban myths about how the birds came to populate parts of the UK.
One rumour stated the birds escaped from the set of the 1951 film The African Queen, filmed in West London, while another claimed rock star Jimi Hendrix released a pair on Carnaby Street, in the centre of the capital.
But Carter said the reality was perhaps somewhat more mundane.
He added: “They almost certainly just escaped or been deliberately released from people who have had them as pets.
“They have been very very popular pets and cage birds for decades now.
“They’ve just got out, bit by bit.”
A British Trust for Ornithology spokesperson said parakeet numbers are steadily increasing and spreading with the species firmly established in and around London and South East England.
The exotic birds made a new home for themselves in Coton Orchard in Cambridge in 2024 after they were sighted for the first time there.
A flock of ring-necked parakeets was also spotted feasting on the wall of a 19th Century barn on a farm near Welwyn in Hertfordshire in January.
The birds have been spotted in various places including Peterborough, and even as far north as Scotland.