Inequalities set to grow as people in the most deprived parts of the country live healthy lives 20 years shorter than the average

Changes to the state pension age will only expand the already yawning gap between rich and poor in Britain, according to an academic study.

Inequalities are set to grow because of the failure to take into account differences in health and life expectancy across the country, says the report from independent think tank the International Longevity Centre – UK and backed by the charity Age UK.

While most people will live to state pension age and beyond, a large proportion are unlikely to get there in good health, especially in more disadvantaged parts of the UK – places like inner city Glasgow, where the healthy life expectancy is just 46.7 years – close to 20 years lower than the national average of 65.

The report, "Linking state pension age to longevity: tackling the fairness challenge", points to variations both by region and social class. One of its authors, Ben Franklin, said: "If increasing numbers of people leave the workforce before reaching state pension age – to care for loved ones or due to poor health – the economic and fiscal benefits of raising state pension age will be lost.

"And with such significant health inequalities across the UK, there is the very real possibility that the vast majority of people from some local authorities will fail to reach state pension age in good health. Improving the quality of life and not just the quantity of life is critical to ensuring that reforms to state pension age are successful in supporting longer working lives fairly."

Although life expectancy and disability-free life expectancy have increased over time, health gaps are widening. The difference in disability-free life expectancy between women born in the most and least deprived areas was 11.6 years in 2001-04. By 2007-10 it had increased to 13.4.

The report also highlights how the disparities could offset the perceived financial benefits of raising the state pension age – moving money saved from pensions into disability and unemployment benefits as ill people fall out of the workforce.

The think tank has called on the government to look at the recent increase in the numbers of employed men and women reporting a long term health problem and to aim health promotion strategies at poorer social classes in an effort to tackle health inequalities.

The government is planning to introduce a flat rate state pension from April 2016 and raise the pensionable age from 66 to 67 between 2026 and 2028."As we live longer lives it appears to be a natural move to raise the state pension age," said ILC-UK's chief executive, Baroness Sally Greengross.

"We need to be very careful to ensure that increasing the state pension age doesn't just result in an increase in the numbers of people out of work and ineligible for state pension. The highest social class are likely to live 13.4 years longer disability-free life than the lowest."