As New Yorkers head for the beach over a public holiday weekend they will be keeping a wary eye on the water amid reports a great white shark has been lurking.

Fears of a "Jaws"-style frenzy erupted after the 3m-long shark, weighting 242kg, was detected just off the coast this week.

The predator, which has been named Cabot, is fitted with an electronic tracking device.

It repeatedly "pinged," apparently placing the shark in Long Island Sound just off the wealthy coastal town of Greenwich, Connecticut, north of New York City.

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Ocearch, a marine monitoring group, set off mild hysteria when it issued a warning about Cabot's presence. A website for the public to track the shark's progress duly crashed under a wave of users.

The group said it was the first time they had detected a large great white shark in Long Island Sound. The last known shark attack in the sound was in 1961.

Beaches in the area opened this weekend ahead of the Memorial Day bank holiday on Monday, the unofficial start of summer.

Ocearch later cast some doubt on its own findings, reporting that Cabot had also "pinged" 12 hours later off the Hamptons, the holiday retreat for many of New York's wealthiest, on the southern shore of Long island.

For both "pings" to be accurate Cabot would have had to swim nearly 321km in half a day.

A spokesman for the monitoring group said: "He either was in the sound, or he was never in the sound. We have calculated that he wouldn't have had time to go all the way around the island."

The uncertainty left beachgoers wondering where exactly the shark might be hunting for its next meal.

But environmentalists, at least, were happy to see it.

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They suggested Cabot's arrival was an indicator that recent efforts to reduce pollution in Long island Sound had been successful.

A spokesman for the Maritime Aquarium in Connecticut said the area was "cleaner and healthier than for years," which may lead to the appearance of "occasional large apex animals".

Other experts said great white sharks may well have started swimming into the sound recently, but this was the first to be noticed because it was fitted with a tracker.

Cabot was tagged off Newfoundland, Canada in October, and named after the 15th Century Italian explorer John Cabot.

It has since swum 6,437km, reaching Florida and the Gulf of Mexico.

The shark was believed to be heading back towards Canada when it took a wrong turn into Long Island Sound.