Last year there were 1,133 sightings of the creatures and researchers at the Marine Conservation Society (MCS) said they have been spotted more than 500 times already this year.
Scientists have attributed the high numbers of sightings to recent mild winters and warm summers.
The figures were released to coincide with a new MCS report detailing more than 5,000 sightings of eight jellyfish species sent to the MCS by members of the public between 2003 and 2011.
It discovered “hotspots” for barrel jellyfish in Scottish and Welsh waters. However, there has been a “remarkable” number of barrel jellyfish spotted off south-west England this year.
Professor Brendan Godley, of the University of Exeter, said: “The remarkable number of barrel jellyfish reported from south-west England this year is quite unusual, and at odds with what our report describes, previous years have seen hotspots for this species in western Welsh and Scottish waters.
“We’re not sure why, but the very mild winter probably meant more adults survived at depth, which will have returned to the surface in spring as waters warmed up. This year’s strange barrel jellyfish results highlight the importance of running the survey yearly.”
The report, the MCS National Jellyfish Survey, takes stock of where and when UK jellyfish occur in UK seas for the first time in more than 40 years.
Scientists said that this year could also be a record-breaking one for jellyfish sightings, with more than 500 reports already received by mid-July.
This year MCS has so far received reports from around the UK of seven of the eight species – including barrel, moon, blue, compass, lion’s mane, mauve stingers and by the wind sailors, which is a close relative of the jellyfish.
Dr Peter Richardson, biodiversity manager for the MCS, said that it is important to learn more about jellyfish because of the impact they can have on a number of sectors, including tourism.
He said: “We still know relatively little about jellyfish, but given the economic impacts that large numbers of jellyfish can have on tourism, fishing, aquaculture and even power generation, we can’t afford to ignore them.”
According to the researchers the moon jellyfish is the most common species, and makes up 29 per cent of the jellyfish population in UK seas. The report was published in the Journal of the Marine Biological Association.