There is a discrepancy in the number of coronavirus deaths reported across the UK as a result of how and when the figures are counted and reported.

Data from the Office for National Statistics (ONS) has been different to that of NHS England and Public Health Wales.

Here is an outline of why that is.

The ONS figures outline deaths where 'Covid-19' was mentioned on the death certificate. Credit: PA Graphics

According to the ONS, a total of 210 deaths in England and Wales that occurred up to and including March 20 (and registered up to March 25) had coronavirus mentioned on the death certificate.

This compares with 170 coronavirus-related deaths reported by NHS England and Public Health Wales, again up to and including March 20.

The numbers suggest 40 "more" deaths took place in this period than previously thought - but why are the numbers different?

The ONS figures - which are provisional - are based on the number of deaths registered in England and Wales where Covid-19 was mentioned on the death certificate as "deaths involving Covid-19".

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The ONS figure includes all deaths - not just those in hospitals - but there is usually a delay of at least five days between a death occurring and registration.

In contrast, figures published by NHS England and Public Health Wales are for deaths only among hospital patients who have tested positive for the virus.

This is something the Department of Health has since started to qualify in its daily tweet with the latest figures.

NHS England and Public Health Wales figures also include deaths that have not yet been registered.

A death is only included in these figures once families of the victim has been informed. This can sometimes take quite a few days - so here too there is a lag between a death occurring and it being counted in the total.

As a result, the day-on-day change in death figures reported by health bodies like NHS England and Public Health Wales is not moving in step with how many coronavirus-related deaths are actually happening in real time.

The variation helps to explain why the numbers are different - but also emphasises the need to be careful when trying to compare the two sets of data.

There may still be deaths that occurred in this period which have yet to show up in either set of figures.

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It has now been confirmed that the true number of coronavirus-related deaths in England and Wales is higher than the number reported by public health bodies.

Because of the difference in how the numbers are compiled, however, we cannot make any projection about how many more deaths are continuing to take place outside of hospitals.

We also cannot be sure of precisely where we are in the "curve" of the coronavirus-related deaths.

Intensive Care Units across Europe are facing strain as the number of virus-cases rises. Credit: AP

This is due to the amount of time that can pass between somebody contracting the virus and their death.

A study published in The Lancet on March 12 based on data reported in Wuhan, China, found that the median average time from the onset of coronavirus symptoms to admission to an intensive care unit (ICU) is about 10 days.

A separate study by the Intensive Care National Audit and Research Centre - based on cases in England, Wales and Northern Ireland up to March 26 - found that those who died spent a median average of four days in an ICU from admission to death.

These factors - the onset of symptoms and the time it takes for a patient to die while in hospital - adds further to the lag in the death statistics.

It means steps taken to limit the spread of the virus, such as social distancing or lockdowns, will take time to show up in the number of deaths.

We are likely to have to wait at least a couple more weeks to get a clear idea of whether the recent measures introduced in the UK to "flatten the curve" of virus-related deaths have had an impact.

Coronavirus: Everything you need to know