The Gulf Stream currents that give Britain its mild climate have weakened dramatically, offering the first firm scientific evidence of a slowdown that threatens the country with temperatures as cold as Canada’s.

The Atlantic Ocean "conveyor belt" that carries warm water north from the tropics has weakened by 30 per cent in just 12 years, scientists have discovered.

The findings, from the National Oceanography Centre in Southampton, give the strongest indication yet that the Europe’s central heating system is breaking down under the impact of global warming.

Scientists have long predicted that melting ice caps could disrupt the currents that keep Britain at least 5C (9F) warmer than it should be, but the new research suggests that this is already underway.

It points to a cooling of 1C (1.8F) over the next decade or two, and an even deeper freeze could follow if the Gulf Stream system were to shut down altogether.

The British Isles lie on the same latitude as Labrador on Canada’s east coast, but are protected from a similarly icy climate by the Atlantic conveyor belt. Its currents carry a million billion watts of heat across the ocean — the equivalent of the output of a million power stations.

Though oceanographers still think it unlikely that the currents will stop completely, this could reduce average temperatures by between 4C (7.2F) and 6C (10.8F) in as little as 20 years, far outweighing any increase predicted as a result of global warming.

Even a lower fall in temperatures could mean Britain gets colder even as the rest of the world warms up, and would severely disrupt the Government’s plans for mitigating the effects of climate change.

The Gulf Stream begins in the Gulf of Mexico and carries warm water north and east, through the straits of Florida and across the North Atlantic. Halfway across the ocean, it branches into two, with one current flowing south towards Africa and another drifting towards northern Europe.

By the time the northern current reaches the Arctic, its waters have become colder and more saline, causing them to sink. A vast undersea river of cold water then flows back towards the Gulf of Mexico, where the process begins again.

Global warming is predicted to disrupt this process, as extra freshwater from melting ice caps and glaciers reduces the salinity of the Arctic waters, stopping it from sinking, and breaking the circuit.

To assess whether this is already happening, the Southampton team measured current flow across a latitude of 25 degrees north. The original Gulf Stream, cold water returning from the Arctic, and the southern branch of warm water all cross this line stretching from north Africa to the Bahamas. Measurements taken in 2004 were compared with data collected in 1957, 1981, 1992 and 1998.

The results, published today in the journal Nature, show that while the outward flow of the Gulf Stream has not changed, the strength of the cold water returning from the Arctic has fallen by 30 per cent since 1992.

Over the same period, the flow of warm water branching off the Gulf Stream towards Africa has increased by 30 per cent. This suggests that the current’s warm waters are being diverted to the south and away from Europe, with potentially serious consequences for the continent’s climate.

"The Gulf Stream splits into two components at a latitude of 40 degrees north, with one flowing south and one into northern latitudes," said Harry Bryden, who led the study. "What we found is that more of the Gulf Stream water is recirculating to the south.

"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."

Meric Srokosz of the Natural Environment Research Council, which funded the work, said: "If it is persistent or there is a further decline, then yes it would have an impact on the climate. The models suggest that if the change is persistent we might see the order of a 1C drop in temperature here over a decade or two. That is assuming the flow stops decreasing.

"The models imply that after 20 years of a total shutdown, temperatures would fall by 4C to 6C in the UK. A German model suggests a 50 per cent slowdown would reduce temperatures by about 2C in northern Europe."

The scientists are not yet certain whether the changes are a direct result of Europe, or whether they will persist. It remains possible that flows change annually or seasonally, and that the 1992 and 2004 data were aberrations.

To determine this, a project is now underway to monitor the currents of the Atlantic continuously for four years. Phil Newton of NERC 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. That is why we are funding continuous monitoring."

The changes in flow appear to have taken place during a decade that has seen the warmest temperatures on record measured in Europe. The discrepancy could be the result of the flow changes being cyclical, or because of a time lag before they start to affect temperatures.

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