Millions of Europeans who arrived late to work or school on Wednesday had a good excuse — an unprecedented lag in the continent's electricity grid that's slowing down some clocks.

The problem is caused by a political dispute between Serbia and Kosovo that's sapping a small amount of energy from the local grid, causing a domino effect across the 25-nation network spanning the continent from Portugal to Poland and Greece to Germany.

"Since the European system is interconnected … when there is an imbalance somewhere, the frequency slightly drops," said Claire Camus, a spokeswoman for the European Network of Transmission System Operators for Electricity.

The Brussels-based organisation, known as ENTSO-E, said in a separate statement that "this average frequency deviation, that has never happened in any similar way in the Continental European power system, must cease".

The deviation from Europe's standard 50 Hz frequency has been enough to cause electric clocks that keep time by the power system's frequency, rather than built-in quartz crystals, to fall behind by about six minutes since mid-January. The problem mostly affects radio alarms, oven clocks or clocks used to program heating systems.

Millions of people in 25 European countries are oversleeping thanks to the power lag. ( Supplied: Pexels )

ENTSO-E said it was working on a technical solution that could bring the system back to normal within "a few weeks", but urged European authorities and national governments to address the political problem at the heart of the issue.

"This is beyond the technical world," Ms Camus said.

"Now there needs to be an agreement between Serbia and Kosovo about this lack of energy in the Kosovo system.

"You need to solve it politically and then technically."

Long-running power dispute

The friction between Serbia and Kosovo is part of a broader dispute that goes back almost 20 years.

Since the war in Kosovo ended in 1999, the Serb-dominated north of Kosovo — which remains loyal to Belgrade — hasn't paid the Kosovo government for the energy it consumes.

A 2015 agreement was meant to resolve the dispute, but Serbia has blocked its implementation.

ENTSO-E warned "if no solution can be found at political level, a deviation risk could remain".

So far the only consequence seems to be the effect on clocks.

"The system is built in such a way that all your basic needs are really secured by the distribution and the transmission system operators," Ms Camus said.

"Frankly, there is no risk other than those clocks running behind."

AP