The Paris Observatory has announced it will add a "leap second" to the last minute of June this year to compensate for a slight slowdown in the Earth's rotation - and the news has the finance world nervous.

The atomic clocks used to keep the planet's official time will read 23:59:60 for one second before switching to midnight, extending June for just one second.

In Australia the extra second will come at 10am (AEST) tomorrow.

It will be the first "leap second" in three years, and it could have unintended consequences for the finance world.

The "extra second" is not counted automatically by computers at the stock markets, meaning it has to be inserted manually and could cause glitches in trading.

The fraction of time also provides more time for electronic trading to take place.

The last leap second occurred in 2012 and affected the Qantas check-in system, delaying up to 50 flights.

However, Qantas has said precautions have been taken to make sure the same won't occur this year, the Sydney Morning Herald has reported.