The number of jellyfish reported in UK waters has soared by 32% compared to last year, The Marine Conservation Society reports.
Increased numbers of rarer types of jellyfish have also been reported in UK seas this year, along with a larger number of 'blooms', of over 100 individuals.
It comes amid a rise in sea temperatures, with global ocean surface temperatures reaching a new record peak of 20.96C in August.
Jellyfish can be found off the British coast year-round, but typically increase in number in spring and autumn when they can appear in large blooms.
This September saw a staggering 400% in jellyfish sightings, compared to the same month last year.
There were a total of 1,737 jellyfish sightings reported to the Marine Conservation Society by the public in the 12 months from last October, to September this year.
Large blooms of over 100 jellyfish accounted for 11 per cent of all sightings - rising by nearly 60 per cent from last year.
The most commonly-reported species was the barrel jellyfish, spotted 467 times. Sometimes called ‘dustbin lid jellyfish’ due to their large size, they can grow up to one metre in diameter.
Barrel jellyfish sightings have risen by 20% from last year, when it was the sixth most-spotted species in the UK.
But rarer species such as crystal and comb jellies rose this year, making up 10% of all sightings.
The compass jellyfish, which was last year’s most-reported species, fell to fourth place, whilst moon jellyfish dropped from second to third place, and lion’s mane jumped from third to second.
A report by the Marine Conservation Society noted the rise and fall in different species as "interesting changes" which "may indicate changes in our ocean".
"Jellyfish populations fluctuate depending on environmental factors, which may be the reason for this change," it said.
"We’ll continue to monitor these trends over the coming year to see what they might tell us about life in our seas."The Marine Conservation Society also received 12 reports of marine turtle sightings, four of which were live leatherbacks.
