James Hansen, director of NASA's Goddard Institute for Space Studies, said that because no direct data was available in the Arctic, analysts had to estimate temperatures from nearby weather stations.

As a result it was not possible to say with certainty that 2005 had been the hottest year yet, but he added: "I'm reasonably confident it was".

Mr Hansen said what was most important was that temperatures in 2005 reached levels of 1998 without the help of the "El Niño of the century".

The analysis confirms a prediction the institute made in December, when the National Oceanic and Atmospheric Administration (NOAA) contradicted it, ranking last year as only the second hottest.

Yesterday, Jay Lawrimore, who heads the NOAA's climate monitoring branch, said his team's current data still suggested 2005 was a close second to 1998. But he said a forthcoming analysis was likely to show 2005 was slightly warmer.

Mr Hansen blamed a build-up of heat-trapping greenhouse gases for making Earth heat up by just over 1F in the past 30 years.

Last month the United Nation's weather agency reported that global temperatures in 2005 had been almost half a degree Celsius warmer than average, with October and June the hottest ever recorded on a global level.

The international conservation charity WWF reported the high temperatures had brought record-breaking drought in the Amazon basin and made sea-ice levels in the western United States and Arctic lower than ever.

Caribbean water temperatures also reached a record high last year, the WWF said.

The news comes after a leading climate expert warned that Scotland would face deadly heatwaves within 50 years.

Professor Paul Wilkinson of the London School of Hygiene and Tropical Medicine, who is to speak at a major environment conference in Stirling next month, said Britain was likely to face "every year or two" the kind of heatwave that killed 35,000 across Europe in 2003.

That year Britain recorded its hottest ever temperature, with a record 38.1C in Gravesend, Kent, while 9 August saw Scotland reach the record temperature of 32.9C at Greycrook in the Borders.

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