The world's all-time heat record lowered a tad on Thursday, as the World Meteorological Organisation declared a phenomenal 58 degrees Celsius (136.4 degrees Fahrenheit) recorded in Libya 90 years ago was wrong.

Instead, the title of the world's hottest place should go to Death Valley, California, it said.

"The all-time heat record held for exactly 90 years by El Azizia in Libya is invalid because of an error in recording the temperature," the UN body said in a statement.

The conclusion comes from a danger-fraught probe last year, conducted in the throes of the Libyan revolution, into how 58 Celsius (136.4 Fahrenheit) came to be documented on September 13, 1922 at El Azizia, southwest of the Libyan capital Tripoli.

A panel of climate experts from around the world found that the thermometer used was not standard and determined that the person who measured the temperature was probably inexperienced.

"We're pretty sure that the person who was tasked with taking the measurements using this instrument didn't know how to use it," Randy Cerveny, the WMO rapporteur on climate extremes who headed the project, said in a video.

In the 1922 logsheet, "the observer had put the numbers in the wrong columns. That kind of tells us that he wasn't used to doing weather observation work," said Cerveny.

He theorised that the unidentified individual had in fact completely misread the thermometer "and was off by five degrees Celsius (8.2 Fahrenheit)."

The committee "decided that this measurement... simply wasn't valid," he said. "It was not the world's hottest temperature."

That honour, the WMO said, has thus been passed to what until now had been considered the next hottest temperature recorded on the planet: 56.7 degrees Celsius (134 degrees Fahrenheit) measured on July 10, 1913 in Death Valley, California.

The investigation "found some pretty startling things" in its detective work, Cerveny said.

And it was also a rollercoaster affair for those who took part in it.

The head of the Libyan National Meteorological Centre under Moamer Kadhafi, Khalid El Fadli, a vital member of the probe and who had dug up documents that shed doubt on the record, went missing for six months during the revolution.

"I, and the rest of the committee, thought El Fadli was a dead man," committee member Christopher Burt said in a posting on the Weather Underground blog.

But El Fadli resurfaced in August 2011, holding the same position for the rebel authorities, and the suspended probe was able to continue.