Climate experts have been forced to admit another embarrassing error in their most recent report on the threat of climate change.

In a background note – released by the Intergovernmental Panel on Climate Change (IPCC) last night – the UN group said its 2007 report wrongly stated that 55% of the Netherlands lies below sea level. In fact, only 26% of the country does. The figure used by the IPCC included all areas in the country that are prone to flooding, including land along rivers above sea level. This accounts for 29% of the Dutch countryside.

"The sea-level statistic was used for background information only, and the updated information remains consistent with the overall conclusions," the IPCC note states. Nevertheless, the admission is likely to intensify claims by sceptics that the IPCC work is riddled with sloppiness.

The disclosure will intensify divisions between scientists and sceptics over the interpretation of statistics and the use of sources for writing climate change reports, disagreements that have led to apologies being made by both sides of the debate. Last week a key climate-change sceptic apologised for alleging that one of the world's leading meteorologists had deliberately exaggerated the dangers of global warming.

In an email debate in the Observer, Benny Peiser, head of the UK Global Warming Policy Foundation, quoted Sir John Houghton, the UK scientist who played a key role in establishing the IPCC, as saying that "unless we announce disasters, no one will listen".

But in a letter to the Observer, Houghton said: "The quote from me is without foundation. I have never said it or written it. Although it has spread on the internet like wild fire, I do not know its origin. In fact, I have frequently argued the opposite, namely that those who make such statements are not only wrong but counterproductive."

Houghton said he was incensed because he believed the quote attributed to him, and to the IPCC, an attitude of hype and exaggeration and demanded an apology from Peiser.

For his part, Peiser told the Observer that he welcomed the clarification. "For many years, the Houghton 'quote' has been published in numerous books and articles. I took Sir John's failure to challenge it hitherto as a tacit admission that the 'quote' was accurate and reflected his view on climate policy. Now that he has publicly disowned the statement, I will certainly refrain from using it."

Houghton's "quote" has become one of the most emblematic remarks supposed to have been made by a mainstream scientist about global warming, and appears on almost two million web pages concerned with climate change. The fact that it now turns out to be fabricated has delighted scientists.

"We do not over-egg the pudding when it comes to the evidence about global warming – and I hope people will now appreciate this point," said Alan Thorpe, head of the Natural Environment Research Council.