The study estimates 18.2 million may have died by the end of December 2021, whereas official tallies record 5.9 million deaths between January 2020 and the end of last year.

The higher figure comes from the first peer-reviewed study of excess deaths (the difference between the number expected based on past trends and the actual number).

It looked at data from 191 countries and territories between January 2020 and December 2021.

UK deaths are roughly what official figures suggest, with the study estimating between 163,000 and 174,000.

Researchers sourced data from government websites, the World Mortality Database, Human Mortality Database, and the European Statistical Office.

It was then used in models to estimate excess deaths. Globally, it was estimated at 120 per 100,000 people.

However, researchers believe 21 countries had excess deaths rates greater than 300 per 100,000.

The highest was Andean Latin America - 512 deaths per 100,000; Eastern Europe - 345 deaths per 100,000; Central Europe - 316 deaths per 100,000; Southern sub-Saharan Africa - 309 deaths per 100,000; and Central Latin America - 274 deaths per 100,000.

However, some countries had fewer deaths than expected from historical trends.

Iceland had 48 fewer per 100,000; Australia 38 fewer; and Singapore 16 fewer.

In sheer numbers, South Asia was the region estimated to have had most excess deaths - 5.3 million; with India having the highest of any country at 4.1 million.

US and Russia excess deaths are believed to be 1.1 million each, according to the study published in The Lancet.

Large differences between official figures and excess deaths could be due to under diagnosis from lack of testing and problems reporting death data, according to the authors.

They also say it is vital to distinguish between deaths caused directly by COVID and those that happened as an indirect result of the pandemic.

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Initial studies suggest a significant proportion of the excess deaths are directly down to the disease. But others may be down to problems accessing healthcare and services - as well as causes such as suicide or drug use.

The authors say the balance will become clearer as countries release more data.

They also acknowledge limitations to their work, such as using a model to predict excess death from countries that didn't report weekly or monthly data.

Lead author Dr Haidong Wang, of the Institute for Health Metrics and Evaluation, said: "Understanding the true death toll from the pandemic is vital for effective public health decision-making.

"Studies from several countries, including Sweden and the Netherlands, suggest COVID-19 was the direct cause of most excess deaths, but we currently don't have enough evidence for most locations.

"Further research will help to reveal how many deaths were caused directly by COVID-19, and how many occurred as an indirect result of the pandemic."