An increase in Jellyfish globally could be linked to climate change, pollution and over-fishing, marine experts have said.
Jellyfish blooms are on the rise following recent warm weather after the cold spring delayed their appearance.
The Marine Conservation Society's (MCS) biodiversity programme manager Dr Peter Richardson said: "The scarcity of jellyfish reports before June was unusual and could well be linked to the exceptionally cold spring.
"However, as our waters warmed, sightings of jellyfish increased, with moon jellyfish reported in large numbers around the UK, reports of compass and blue jellyfish in the South West, and blooms of lion's mane jellies around North Wales and north-west England."
And he said: "There is some evidence that jellyfish numbers are increasing in places around the world, including UK seas, although some scientists argue that jellyfish numbers increase and then decrease normally every 20 years or so.
"However, others believe these increases are linked to factors such as pollution, over-fishing and possible climate change.
"I think we should consider jellyfish populations as important indicators of the state of our seas, and the MCS jellyfish survey helps provide some of the information we need to understand more about them."
The MCS is urging people to report their sightings of jellyfish, which act as a barometer of the seas, as part of its annual national jellyfish survey.
This year they had been a rare sight in UK seas until hot weather warmed coastal waters in recent weeks. Now, increasing numbers of moon, compass, blue and lion's mane jellyfish have been reported.
The charity warned that lion's mane jellyfish have a powerful sting and anyone taking part in the survey should look but not touch jellyfish that they see.
The survey for the UK, which asks people to report jellyfish they see in the sea or on beaches, comes after a mass invasion of thousands of miles of the Mediterranean coastline by millions of jellyfish in June, affecting tourists who head there in the summer.
Dr Richardson said: "We still know relatively little about jellyfish, but large increases in numbers like those in the Mediterranean are telling us about the health of our seas and cannot be ignored."