The barred grass snake, Natrix helvetica, is now recognised as a species in its own right distinct from the common or eastern grass snake (Natrix natrix).
Both snakes can be found in lowland areas of southern England.
Unlike the adder (Vipera berus), neither creature is venomous.
The only other British snake, the smooth snake (Coronella austriaca), is also non-venomous and extremely rare.
Grass snakes, which grow to more than a metre (3ft) in length, live near water mainly feeding on amphibians such as frogs, toads and newts.
The barred grass snake is greyer in colour than its olive green cousin and lacks its most striking feature, a bright yellow collar.
Along the body are dark bands that are much more pronounced than they are in the common grass snake.
It used to be thought there was just one overall species of grass snake plus a number of subspecies that varied in appearance.
Professor Uwe Fritz, from the Senckenberg Research Institute in Germany, who led the international team of researchers, said: "We studied two areas where different genetic lineages of the grass snake come into contact.
"We discovered that the barred grass snake, previously considered a subspecies, is in fact a distinct species.
"The barred grass snake is widely distributed throughout Great Britain, Switzerland, Italy and France, and also occurs in the western part of Germany.
"Thus, the number of European snake species has increased by one."
The scientists looked at the genetic identity of more than 1,600 grass snakes, including many specimens in museum collections.
Grass snakes in two "contact zones", where different genetic lineages of the animals met, were studied in more detail.
One zone was in the Rhine region while the other extended from central Germany to the southern Balkans.
The scientists found evidence of reproductive barriers that indicated the barred grass snake was a separate species.
The discovery, described in the journal Scientific Reports, could have implications for grass snake conservation, said the researchers.
In the UK, the grass snake is protected under the Wildlife and Countryside Act, making it a criminal offence to injure or kill the animals.
"We now have to pay close attention to which species of grass snake is involved in each case, in order to be able to assess whether one of them may be more threatened than previously thought," said Prof Fritz.