Deaths in England and Wales hit a 12-year high last year, triggering a drop in life expectancy for men and women.
There was a surge in deaths between 2014 and 2015, with flu thought to be to blame for taking the lives of a significant number of people aged 75 and over.
As a result, life expectancy at birth fell by 0.2 years for men and 0.3 years for women between 2014 and 2015.
Life expectancy fell to 79.3 years for men and 82.9 years for women, analysis by the Office for National Statistics (ONS) shows.
Across England and Wales, there was a rise of 28,189 deaths in 2015 (up 5.6%), from 501,424 deaths in 2014 to 529,613 deaths in 2015.
This is the highest number since 2003, when there were 539,151 deaths. The percentage increase in 2015 is the largest year-on-year rise since 1967 to 1968 (6.3%).
The vast majority of the extra deaths in 2015 were registered in the first three months of the year.
There were 24,065 more deaths in the first three months of 2015 compared with the same period in 2014, with 11,865 of these extra deaths registered in January alone, when flu was circulating at its highest levels.
The ONS report showed there were 24,201 extra deaths in the 75 and over age group in 2015 - accounting for 86% of the total increase.
Dr Richard Pebody, head of flu surveillance for Public Health England, which helped conduct the research, said: "A(H3N2) was the dominant subtype circulating last flu season in the UK.
"In any flu season dominated by an H3 subtype, unfortunately we can expect the burden of illness to be seen in the elderly due to its intense nature.
"Winter 2014/15 was unusual because the circulating flu strain also drifted, making the vaccine 34% effective, lower than the typical 50% we had seen in recent previous years.
"It is possible that this exacerbated the situation further, leading to increased levels of excess mortality compared to recent seasons.
"The flu vaccine is updated every year and it is crucial that we remember that vaccination remains the best protection we have against the unpredictable flu virus."
The ONS report showed there was a rise in the number of people whose death certificate said the underlying cause was dementia or Alzheimer's (accounting for 41% of extra deaths among over-75s), although a third of these also had respiratory disease, such as flu, mentioned.
Overall, 31% of death certificates said there was an underlying cause of respiratory disease, although the ONS said flu was generally under-reported so the figure was likely to be higher.
Claudia Wells, head of mortality analysis at ONS, said: "The majority of the increase in deaths in 2015 happened during the first few months of the year, coinciding with an increase in hospital admissions for flu and reports of numerous outbreaks of the virus in care homes."
Professor John Newton, chief knowledge officer at Public Health England, which worked on the analysis, said: "A range of factors can push up the number of deaths in older people in a particular year.
"An outbreak of flu can have a big impact, especially on those who are most vulnerable or experiencing other illnesses, such as dementia."
Jeremy Hughes, chief executive of Alzheimer's Society, said: "With one in six deaths in England and Wales of people over the age of 75 now attributed to dementia, these findings serve as a stark reminder of the need for good community care to support the most vulnerable.
"People living with dementia often have a lowered immune system and so are at a greater risk of contracting flu viruses. The condition also makes it harder for people to look after themselves and in the cold winter months this can become a real danger."