The pair of police community support officers raised the alarm after spotting the cobra curled around the handlebars of a bicycle.
They cleared the area around Thorpe Bay Yacht Club, in Southend-on-Sea, Essex, while they called in expert help.
Staff at the Essex Police control room even rang up specialist Iain Newby, owner of the Dangerous and Wild Animal Rescue Facility in nearby Great Wakering.
But when he arrived at the scene on Sunday lunchtime, the red-faced officers revealed it was a rubber toy.
Mr Newby said: “I grabbed all the kit I would need to protect myself from a venomous snake, including a snake hook and my snake stick, which I need to grab snakes to stop them biting.
“I spotted the officers beside the yacht club boat yard when they said it was a rubber model.
“I must admit I laughed and asked if I could keep the snake.”
Despite the blunder, Mr Newby praised the PCSOs for their quick-thinking.
He said: “More and more people are keeping things like this as pets and it’s not impossible for them to get loose.
“So they did exactly the right thing while they were not sure what they are dealing with, as even rubber snakes are getting more and more accurate.”
Real-life cobras, or ‘hood-snakes’ can have deadly venom and grow to 2 metres long.
PCSO Tristan Adams, of Essex Police, said the toy was “very realistic”.
He said: “We will always err on the side of caution when dealing with incidents such as this, which could potentially pose a significant threat to members of the public.
“It was only a very realistic replica snake, but I’d rather we’d over-reacted to a false alarm than ignored a real emergency.”