Deaths in England and Wales have risen sharply above what would normally be expected at this time of year.
The Office for National Statistics data showed there were 18,500 fatalities in the week up to 10 April – around 10,000 deaths a week would be expected.
More than 6,200 were linked to coronavirus, a sixth of which were outside of hospital.
But deaths from other causes also increased, suggesting the lockdown may be having an indirect impact on health.
Nick Stripe, head of health analysis, said they were trying to understand what had been happening with this increase in non-coronavirus deaths.
He said it could be that people with other illnesses were avoiding going to hospital treatment – visits to A&E have halved since the pandemic started.
But he added it could “take years to work out”.
Mr Stripe said the overall number of deaths for the latest week – 18,500 – was the highest number for 20 years.
“Each one is a person. Each one has a family. We must always remember this.”
A similar trend is being reported in Scotland where there were nearly 2,000 deaths in the week up to 12 April.
Deaths in Northern Ireland are also up.
How many are dying from coronavirus?
This data is different from the daily death figures which are announced by government.
That looks at deaths in hospital where a person is infected with coronavirus, which does not tell us to what extent the death is caused by the virus.
The ONS figures rely on death certificates where the cause or contributory factors are listed.
As a result, the figures lag behind the government figures but give a more accurate picture of what is happening.
They cover deaths in all situations – care homes and the community as well as hospitals.
They show that overall a third of deaths during the week up to 10 April were linked to coronavirus.
Since the pandemic started, more than 1,000 deaths have been seen in care homes.