JOIN USStudies of the NPL's CsF2 clock's performance, to be published in the journal Metrologia, show it is nearly twice as accurate as previously thought.
The clock would lose or gain less than a second in some 138 million years, the BBC reported today.
The UK is among the handful of nations providing a "standard second" that keeps the world on time.
However, the international race for higher accuracy is always on, meaning the record may not stand for long.
The NPL's clock is a "caesium fountain" atomic clock, in which the "ticking" is provided by the measurement of the energy required to change a property of caesium atoms known as "spin".
The NPL-CsF2 clock provides an "atomic pendulum" against which the UK's and the world's clocks can be compared, ensuring they are all ticking at the same time.
That correction is done at the International Bureau of Weights and Measures (BIPM) in France, which collates definitions of seconds from six "primary frequency standards" - CsF2 in the UK, two in France, and one each in the US, Germany and Japan.
For those six high-precision atomic pendulums, absolute accuracy is a tireless pursuit.
At the last count in 2010, the UK's atomic clock was on a par with the best of them in terms of long-term accuracy: to about one part in 2,500,000,000,000,000, the report said.