The snake, found slithering along near woodland in the town’s Bury Field, measured at least 4ft (1.1m).
The average size for grass snakes in this country is around 60cm for a male and 90cm for a female.
Grass snakes are semi aquatic and feed mostly on toads, frogs and newts. They will occasionally take prey such as small mammals and occasionally, birds.
They are extremely strong swimmers and favour grasslands and open woodlands with water nearby. They have sometimes been spotted in domestic gardens, particularly those that contain a pond.
The markings of grass snakes vary and they can be anything from olive-green to brown in colour and have a a yellow and black ‘collar’, which is most obvious in juveniles or adults that have recently shed their skin.
Grass snakes have black bars down their flanks, which sometimes leads people to confuse them with an adder.
Adders are greyish with a distinct zig-zag pattern down their backs. Males are more silvery-grey, while females are lighter or reddish-brown.
The adder is the UK's only venomous snake, but, according to the Wildlife Trusts, its poison is generally of little danger to humans. An adder bite can be painful and cause inflammation, but is really only dangerous to the very young, ill or old.
However, if bitten, medical attention should be sought immediately.
One member of public who spotted the grass snake in Bury Field said: “I’ve seen little grass snakes before but never one as long as this… It was quite scary until I realised it was absolutely harmless!”