A spike in the number of deaths in England and Wales in 2015 - the highest in a decade - was driven by increased mortality in over 75s.

The Office for National Statistics said its 2015 provisional data showed the highest number of deaths in a single year since 2003 and the highest year on year percentage increase since 1968.

Early analysis shows the largest rise in the number of deaths in over a decade was a result of an increase in dementia and Alzheimer’s related deaths and respiratory diseases (including flu) among older people, the ONS reported.

The predominant circulating flu virus in 2015 was influenza A(H3N2) - a strain known to predominantly affect older people - and there were numerous reported outbreaks in care homes, admissions to hospital and intensive care for flu higher than recent seasons.

There was also evidence that the flu vaccine was less effective than expected. The peak in influenza admissions to intensive care occurred in January, at the same time as the peak increase in deaths.

For the first time the ONS, with support from Public Health England, carried out analysis of the weekly and monthly death figures to provide insight.

Last year there were 529,613 deaths registered in England and Wales, an increase of 28,189 (5.6%) compared with 2014, with 86 per cent of the extra deaths occurring in the over 75s and 38% in the over 90s.

The figures also suggested that life expectancy at birth would fall by 0.2 years to 79.3 years for boys and by 0.3 years to 82.9 years for girls if mortality rates remain the same as they were in 2015.

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. Respiratory diseases, such as flu, were also mentioned in a third of deaths from dementia and Alzheimer’s last year”.

“The number of deaths where dementia and Alzheimer’s were listed as the underlying cause have been steadily increasing over the last 15 years, but were well above the 5 year average in 2015.”

Professor John Newton, chief knowledge officer at Public Health England, said: “The population is ageing and we are seeing more people diagnosed and living with illnesses like dementia and Alzheimer’s disease in later life.

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.

“An increase in deaths will generally lead to a decrease in life expectancy that year, but we have seen these annual fluctuations before and the overall trend has remained positive."

Deaths remained close to the 5-year average in the first 12 weeks of 2016, and the large increase in deaths in the early part of 2015 was not repeated in the early part of 2016, the ONS added.