Photographer Steve Trewhella captures the moment he and his grandson encounters giant jellyfish off the coast of Dorset, England. (Steve Trewhella/SWNS)

As thousands of "giant" jellyfish as big as 3-feet wide were reported swarming off the coast of Dorset in southwest England, a photographer captured the moment he and his grandson encountered one of the creatures, almost 5 feet long.

Steve Trewhella, a conservationist and underwater photographer, said he was surprised when he discovered the waters a mile offshore Kimmerdige in Dorset filled with the 5-feet long, 3-feet wide barrel jellyfish. Trewhellaswam among of them, he told The Daily Mail.

"We never expected to stumble across more than we knew what to do with," he told the SWNS media agency. "They were wonderful and such magnificent animals, and you just can't help but be impressed when you see a 4-feet long jellyfish by your side."

Trewhella was snorkelling with his 10-year-old grandson Finn when they saw the 4ft-long barrel jellyfish. (Steve Trewhella/SWNS)

Rhizostoma pulmo, commonly known as the barrel jellyfish, is found in the northeast Atlantic, and in the Adriatic and Mediterranean Sea. They are commonly , according to the Marine Life Network. One of their identifying features is their thick, dome-shaped bell, that becomes thinner around its margin.

Barrel jellyfish can grow up to 5-feet long, and are the largest jellyfish species found in south west England, but they are not considered dangerous. "to us in spite of their size," Cathy Lucas, professor of marine biology at the National Oceanography Centre, Southampton, England, told The Guardian. "All jellyfish sting, but the barrel sting is weak."

Barrel jellyfish, which are harmless, can grow up to 5ft, making it the largest jellyfish in British waters. (Steve Trewhella/SWNS)

In recent years, there have been a noted increase in jellyfish numbers in the UK seas. The increase has been linked to, according to the Marine Conservation Society.

Most jellyfish are , when conditions are just right for jellyfish to feed along the coast of Dorset, says the Dorset Wildlife Trust. Warmer sea temperatures helpcreate an increase of microscopic plants and animals, known as the spring plankton bloom, the main diet for many marine species, creating perfect conditions for jellyfish to flourish.

The Marine Conservations Society says people can expect to see more barrel jellyfish if the warm weather continues.

According to the Marine Conservation Society, more jellyfish blooms are expected as the UK's seas warm up over the summer. (Steve Trewhella/SWNS)

Trewhell, who has been taking underwater photographs for 30 years, told the SWNS Media Agency he believes there are thousands of the 100lb creatures just a few miles from the Dorset coast.

His photos show the barrely jellyfish bigger than it actually is but a shows the jellyfish appearing almost as big as him.

Trewhella says he think this year's jellyfish bloom is even bigger this year. He saw at least 100 jellyfish off the British coast, compared with one or two last year. (Steve Trewhella/SWNS)

Experts say the jellyfish's stings are not powerful enough to do any serious harm, but warn swimmers that , says The Daily Mail.

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