The Independent: The two men boarding the Eurostar to Paris this Wednesday will be unremarkable, save for the metallic suitcase they keep in their sight at all times. Their fellow passengers could be forgiven for shuffling away as they eye the words DO NOT DROP in red on the side of the case.

Far from transporting some doomsday device, the two British scientists will be carrying a kilogram of harmless metal. But not just any kilogram: it is the UK standard kilogram, used as the ultimate reference for everything from the accuracy of a grocer's scales to the ingredients in a pill.

Actually, it weighs a fraction more than a kilogram. And that is a problem: as the years go by, it is gaining weight. Scientists from Britain's National Physical Laboratory (NPL) hope that cleaning the weight with a revolutionary new method, invented in the UK, will bring it back closer to its true mass. Long-term comparisons suggest that other nations' benchmark kilos are also gaining weight compared with the prototype in Paris – the definitive kilo based on the weight of a litre of water at 4C – which was cast in 1879.