There was a spike in deaths in England at the start of this summer's heatwave, new data reveals.
But statisticians say it is too early to tell how many of them can be attributed directly to the heat.
On 25 and 26 June, when temperatures soared, there were 259 more deaths than the five-year average of 2,146 for those two days.
It coincided with temperatures rising above the level at which experts say heat-related deaths can occur.
The Office for National Statistics, which released the data, says it still needs to look at figures for July and August, as well as more information about registered causes of death, before assessing the reasons behind the rise.
The summer was the joint-hottest on record for the UK as a whole and the hottest ever in England, with the temperature peaking at 35.3C (95.5F) in Faversham, Kent, on 26 July.
A heatwave can affect anyone, but the most vulnerable are older people, babies and young children and people with pre-existing health problems.
On 21 and 22 June there were 214 fewer deaths than the five-year average for the same period, the quarterly report by the Office for National Statistics for the period from 1 April to 30 June shows.
But when the temperature climbed into the high 20s and a heatwave alert was issued by Public Health England on 25 June, the number rose to above the average.
There were 1,205 deaths on that day and 1,200 on the following day, 26 June 2018. That compared with an average of 1,072 and 1,074, respectively for the same time of year in the previous five years (2013-2017).
Maximum temperatures exceeded 24.5C - the level at which Public Health England warns that excess heat-related deaths may begin to become apparent - from 25 June, when a high of 30.1C (86.1F) - the UK's hottest of the year at that point - was recorded in London.
During that period, there were 382 more deaths than the average for the same period from 2013 to 2017.
A peak and fall in the number of deaths was also recorded in April, coinciding with another spell of hot weather.
There were 243 more deaths in England between 18 and 19 April than the five-year average across the same period, but 378 fewer between 21 and 23 April.
The ONS quarterly report covers deaths registered in England between April and June.
Overall, there were 108,537 deaths in England during the three-month period - 497 fewer than the five-year average.
The next release, expected in December, will examine in detail those that might be attributable to the 2018 heatwave.
Annie Campbell, an ONS statistician, said: "Although the provisional data currently available appears to show a high number of deaths at the end of June, we can't confirm the heatwave is the cause.
"The question of deaths which might be attributable to a heatwave will be addressed in our next quarterly release."