By the week ending December 18, deaths for this age group were 7.7% higher than the equivalent average figure for the years 2015 to 2019.
The individual rate for England stood at 8.7%, higher than the figures for Scotland (7.7%), Wales (5.0%) and Northern Ireland (4.1%).
For deaths among all ages, Poland ended 2020 with the greatest cumulative excess mortality (11.6% above the five-year average), followed by Spain (10.6%) and Belgium (9.7%).
England ranked seventh on this list (7.8%) with the UK eighth (7.2%).
This is a change from the middle of 2020, at which point England had the highest cumulative mortality rate for countries where data was available (7.3% above average) followed by the UK as a whole (6.7%) and Spain (5.9%).
The figures show how different parts of Europe experienced contrasting levels of mortality throughout 2020, with central and eastern European countries being worse hit in the second half of the year.
Cities of central and eastern Europe suffered particularly high excess mortality in the autumn and winter period, with Sofia (112.5%) and Warsaw (103.8%) having the highest weekly excess mortality rates.
But these were much lower than those cities affected during the spring of 2020, such as Madrid (452.0%), Barcelona (266.0%) and London (228.4%).
The best way of comparing levels of mortality internationally is by looking at all-cause mortality rates – by local area, region and country – compared with the five-year average, the ONS said.
All-cause mortality avoids the problem of different countries recording Covid-19 deaths in different ways, and also takes into account the indirect impact of the pandemic, such as deaths from other causes that might be related to delayed access to healthcare.
Dr Annie Campbell, of the ONS health analysis and life events team, said: “While the UK may no longer have one of the highest levels of cumulative excess mortality in Europe, it does persist to have some of the highest cumulative excess mortality rates for those aged under 65 years.
“Only Bulgaria had a higher cumulative excess mortality rate for this age group by the end of 2020, with the UK and its constituent countries having excess mortality levels well above most other European countries.
“This has been a pattern observed throughout 2020 since the Covid-19 pandemic began in March, showing that the impact of the pandemic in the UK has not exclusively affected those at the oldest ages. We are working to better understand the reasons behind this trend.”