The official kilogram - a cylinder-shaped hunk of metal - that defines the fundamental unit of mass, has itself gained weight, scientists say.
Scientists at Newscastle University have found that due to surface contamination the original kilogram is likely to be tens of micrograms heavier than it was when the first standard was set in 1875.
As a result, each country that has one of these standard masses has a slightly different definition of the kilogram, which could throw off science experiments that require very precise weight measurements or international trade in highly restricted items that are restricted by weight, such as radioactive materials.
The original kilogram - known as the International Prototype Kilogram or the IPK - is the standard against which all other measurements of mass are set.
Stored in the International Bureau of Weights and Measures in Paris, 40 official replicas of the IPK were made in 1884 and distributed around the world in order to standardise mass.
Now Professor Peter Cumpson and Dr Naoko Sano have used cutting-edge X-ray Photoelectron Spectroscopy (XPS) to analyse surfaces similar to the standard kilogram to assess the build-up of hydrocarbons - and how to remove them.
Ozone and ultraviolet light could be used to clean the kilograms without damaging them, LiveScience reported.
"Statute decrees the IPK is the kilogram," explained research lead Peter Cumpson, Professor of Micro Electro Mechanical Systems (MEMS) at Newcastle University.
"It doesn't really matter what it weighs as long as we are all working to the same exact standard - the problem is there are slight differences. Around the world, the IPK and its 40 replicas are all growing at different rates, diverging from the original," Cumpson said.
"We're only talking about a very small change - less than 100 micrograms - so, unfortunately, we can't all take a couple of kilograms off our weight and pretend the Christmas over-indulgence never happened," Cumpson added.
"But mass is such a fundamental unit that even this very small change is significant and the impact of a slight variation on a global scale is absolutely huge. There are cases of international trade in high-value materials - or waste - where every last microgram must be accounted for," he said.
"What we have done at Newcastle is effectively give these surfaces a suntan. By exposing the surface to a mixture of UV and ozone we can remove the carbonaceous contamination and potentially bring prototype kilograms back to their ideal weight," he added.
Work is underway internationally in several National Measurement Institutes to find an alternative to the IPK - a standardised value for the kilogram that is not based on a matchbox- sized piece of metal.
The study was published in the journal of Metrologia.