The latest data from TPR showed that in October 9.9m workers were automatically enrolled into a pension scheme while 11.4m were already active members of a scheme and 438,000 were members of a defined benefit or hybrid scheme.
This left 9.2m - or nearly 30 per cent of all workers at employers which had confirmed to TPR they had complied with auto-enrolment - has not falling into any of these categories, meaning they either weren't saving into a scheme or did not qualify because of their age.
Helen Morrissey, pension specialist at Royal London, said: "If we really want to tackle the pension savings gap then the government really needs to make moves to increase the scope of auto-enrolment so it can include groups such as the self-employed as well.
"These figures show that auto-enrolment works but now we need to make sure it works for more people."
Fiona Tait, technical Director at Intelligent Pensions, said: "The fact that we are discussing it is incredible positive.
"Auto-enrolment, when it was first introduced wasn’t thought of as something valuable enough to be upset at missing out on so those that weren’t included didn’t have strong objections to not being included. Now they are aware that they are missing out.
"It isn’t is fair to say that the Government has ignored these groups, but what we can say is that now auto-enrolment has proven to be a success, something similar should be rolled out for the likes of the self-employed."
Last year, Royal London and Aviva joined forces to produce a paper on how the self-employed could be brought into auto-enrolment.
The paper found this group was "amongst the most under-pensioned group in society" and recommended that default pension saving was made through the tax return to enable those in self employment to save for their retirement.
The report, published in June 2017, stated: "Millions of self-employed people file an annual tax return and Aviva and Royal London believe this could provide an opportunity to nudge them into pension saving. The key is to make the ‘default’ position contributing into a pension rather than not contributing into a pension."
Adrian Lowcock, head of personal investing at Willis Owen, agrees there was a huge challenge in getting people who were self-employed to contribute to a pension.
He said: "The reason these have been left out in previous rounds of legislation is that it is a complex situation. Each self-employed individual is likely to run their affairs in a slightly different way and will fill out a tax return differently too.
"Making the pension contribution included in the tax return it would seriously complicate the issue as it means the individual needs to have to put money aside for that as well as paying any tax due. It will also allow for individuals to be able find ways to possibly avoid making the contribution.
"It is important to remember that self-employed may often need to have significant cash-flow to manage how they do business and conduct their lives and it is in no way a one size fits all situation.
"Education is an important part of the overall process and this route should be explored as well, although of course people can still ignore it. The question is should these individuals be forced to make pension contributions or just encouraged. Auto-enrolment is after all an opt-out process. Of course there is an argument that even a poor system is better than no help at all."
