Scientists have found clear evidence that the ocean current "engine" helping to drive the Gulf Stream is weakening.
Disruption to the conveyor-belt mechanism that carries warm water to Britain's shores was the basis of the Hollywood disaster movie The Day After Tomorrow.
In the film, a breakdown of the system caused by global warming triggers a sudden cataclysmic ice age.
Scientists say such predictions are fantasy - but the real evidence does point to a possible 1C cooling over the next two decades.
More alarmingly it may indicate the beginning of an accelerating change with more severe consequences.
Climate models suggest that if the conveyor ground to a halt - as more extreme forecasts predict could happen - average temperatures could drop by up to 10C in Britain and north-west Europe.
The conveyor, technically known as the Atlantic meridional overturning circulation (MOC), carries warm shallow water into northern latitudes and returns cold deep water southwards across the equator.
It is part of a complex ocean current and atmospheric system responsible for the warm Gulf Stream, which produces as much energy as one million power stations and ensures that Britain enjoys a mild climate.
Without the Gulf Stream, Britain and other parts of Europe at the same latitude would be as cold as Canada.
Experts have expressed the fear that melting polar ice pouring fresh water into the sea might upset the MOC mechanism.
This is because fresh water does not sink as easily as salt water, especially at cold temperatures. As a result less deep cold water flows back along the bottom of the conveyor, and the whole system slows down.
The new findings from British and US scientists, published in the journal Nature, show that the MOC has weakened by 30% since 1957.
Sensors attached to an array of 22 buoys strung across the Atlantic detected the change.
Each mooring is anchored to the seabed on wires up to 5,000 metres long carrying instruments which measure salinity, temperature, pressure and currents.
The £20 million research programme, called Rapid, is co-ordinated by scientists at the National Oceanography Centre at the University of Southampton.
Readings from the sensors were taken during an expedition last year. These were compared with records from 1957, 1981, 1992 and 1998.
Professor Harry Bryden, one of the scientists leading the project, said: "The Atlantic meridional overturning circulation, sometimes called the Conveyor Belt, carries warm upper waters to northern latitudes and returns cold deep waters southward across the equator. It is a massive system that includes the Gulf Stream and it carries heat northward out of the tropics into the northern Atlantic, warming the atmosphere and helping to provide northern Europe with a moderate climate.
"In previous studies over the last 50 years, the overturning circulation and heat transport were reasonably constant. We were surprised that the circulation in 2004 was so different from previous estimates."
Although no change was seen to the northward flow of warm water near the ocean surface, the movement back of deep cold water had reduced significantly.
This indicated that the overall circulation system was slowing. Measurements of water recirculating in subtropical regions showed that less water was completing a "full circuit" of the system.
Ocean flow is measured in units called Sverdrups (Sv), equivalent to one million tonnes of water per second.
The scientists estimate a decrease in the conveyor's "overturning" from 20 Sv recorded in earlier surveys to 14 Sv.
Commenting on the findings, Dr Phil Newton from the Natural Environment Research Council, which is funding the work, said: "This paper shows really tantalising evidence that there may be a large change in the ocean oscillation under way which, paradoxically, could cause a cooling over Europe."
The researchers predict that if the 30% weakening persists, it may lead to a 1C fall in temperature over northern Europe in the next 20 years.
Further disruption to the system could be expected to have greater effects.
But there was no chance of a sudden freeze, as portrayed in The Day After Tomorrow.
Dr Meric Srokosz, another member of the National Oceanography Centre team, said: "In terms of science, you can't cool the whole of the upper hemisphere in 10 days and cause a new ice age. That's a physical impossibility."
He admitted it was harder to predict what might happen in the long term if the conveyor system broke down.
"We don't know - the climate model suggests that if you slow the circulation you can get a local cooling around the north Atlantic, but you're not going to get another ice age," he said.Reuse content