"The risk is less than being killed by a bee sting.

Elin Davies of RSPCA Cymru Wales

Notices have been posted around the area in Rhiwbina in Cardiff, warning people that the snakes are timid and should be left alone.

The common adder is Britain's only venomous viper but it is less aggressive than most snakes and its bite is rarely fatal to humans.

"If they are left alone they're not a threat to anyone," said Elin Davies of RSPCA Cymru Wales.

"The risk is less than being killed by a bee sting.

"If people are bitten they are advised to seek medical attention and if pets are bitten, especially around the mouth, they should see a vet."

Posters have been placed around the area

And as part of a joint poster initiative between South Wales Police and Dwr Cymru, Welsh Water, it is stressed to members of the public that bites are rare.

Statistics show that over a 100 year period (1876 to 1976) only 14 people died as a result of adder bites.

Welsh Water's environment and education manager, Jeff Morgan said: "We have to balance public safety with upholding the law which states that adders are a protected species.

"We want local people to know that these snakes are living on site.

"The expert advice is that if you don't bother adders then they will not trouble you."

The common adder grows to lengths of 65 cm (25.5 in) and occasionally reaches 90 cm (35 in).

Adult males are usually pale grey with a contrasting black pattern of dorsal zigzag stripes.

Females on the other hand may be brown or reddish-brown with a dark brown pattern of zigzag stripes.

People visiting the reservoir are being warned to keep an eye out for the snakes but not to approach or injure them.