Inflation rose to 0.6% last month as clothing prices edged higher following extended Black Friday discounting the month before.
The consumer price index (CPI) measure of inflation was up from 0.3% in November and a little higher than the expected figure of 0.5%.
Clothes prices increased fractionally during December over a period when they usually fall.
Price movements “have been unusual” in 2020 “and appear to have been affected by the impact of coronavirus”, the ONS said.
“The overall price rises in December 2020 follow increased discounting in November 2020, as Black Friday sales may have spread further across the month,” it added.
Also contributing to the uptick in inflation were higher air fares – despite travel restrictions – as well as increased petrol prices, according to the ONS.
Jonathan Athow, ONS deputy national statistician for economic statistics, said: “Clothing prices put upward pressure on inflation in December, despite some evidence of continued discounting.
“Transport costs, including air, sea and coach fares, as well as petrol prices, rose as some travel restrictions eased during parts of the month.
“These were partially offset by falling food prices, most notably for vegetables and meat.”