One in seven people have no plans to retire and increasing numbers of women have no pension, research by Barings Asset Management has found.
Its annual survey of non-retired adults also found that 34% of respondents had no pension.
Among those who did have specific retirement plans, the age at which they expected to stop work had risen to 64, from 62 recorded in the 2009 survey.
The 14% of people with no plan to retire was equivalent, if extrapolated across the working age population, to 5.1 million people, the highest such figure since the Barings' annual study began in 2008 and up two percentage points since 2012.
Further uncertainty about retirement planning was indicated by 40% of respondents saying they did not know when they would be able to retire, up from 38% in 2012 and 36% in 2011.
Barings chief investment officer Marino Valensise said: 'It is clear that uncertainty is increasing for many people around retirement plans, with more people unable to say exactly when they plan to retire and the average age continuing to rise.
'Significant numbers of people nearing traditional retirement age reported they did not know when they will be able to retire and the worry is that a lack of sufficient financial planning and pension provision combined with increased longevity is having a real impact on a large proportion of the population.'
The research found that 34% of respondents - the equivalent of 12.5m across the population - lacked a pension.
Although this figure has remained roughly flat since Barings began its annual survey in 2008, there were substantial differences between groups within this total.
While the proportion of men with no pension has steadily declined, from 30% in 2008 to 26% in 2013, the proportion of women has risen, to 42% from 39%.
In the two highest 'A' and 'B' social groups only 18% of people lacked a pension, but this rose to 61% among the lowest 'D' and 'E' groups.
Valensise said: 'We are alarmed that so many people who should have proper plans in place do not, and urge everyone to better understand the benefits that they can get from planning ahead and starting early.'