For 121 years it has provided a physical basis for scientific definitions of mass and energy.

But now scientists are losing faith in the one kilogramme metallic artefact known as the “international prototype”.

Experts meeting at the Royal Society in London are discussing new research to move closer to the first non-physical definition of the kilogramme.

If this is achieved – possibly within the next five years – it will mark a historic change, the scrapping of the last manufactured object on which fundamental units of measurement depend.

The international prototype is a small cylinder of plutonium-iridium alloy, just under four centimetres tall and wide, whose mass defines the kilogram.

Since 1889 it has remained triple-locked under two glass dome bell jars in a vault at the International Bureau of Weights and Measures on the outskirts of Paris.

Many other units of scientific measurement rely on the international prototype providing the “gold standard” definition of mass.

For instance, the newton is defined as the force necessary to accelerate one kilogramme at one metre per second squared. In turn the pascal, a unit of pressure, is defined in terms of the newton.

However, measurements made over the last 100 years indicate that the international prototype has lost weight.

For reasons that still remain a mystery the cylinder may have shed around 50 micrograms, the equivalent of a small grain of sand.

Scientists are, therefore, searching for a non-physical way of defining the kilogramme. This would bring it in line with the six other base units that make up the International System of Units – the metre, the second, the ampere, the kelvin, the mole and the candela – none of which are now based on a physical reference object.