The UK government expects one million people to opt-out of workplace pension auto-enrolment in 2019, according to analysis by Hargreaves Lansdown.
Once self-employed and non-eligible workers are included in the figures, a total of 13 million people are expected to be outside of pension saving by that time.
This will equate to 27.5% of members opting out of auto-enrolment by 2019, rising from 21.7% in 2018, and an estimated 10% today, with incoming contribution hikes thought to be responsible.
"The government workings point to soaring opt-out rates," Hargreaves Lansdown senior pension analyst, Nathan Long, said. "Auto-enrolment has so far boosted numbers saving, but two huge obstacles are looming in the form of contribution hikes in 2018 and 2019.
"The £18 that someone with full-time average earnings is currently required to spend on pensions each month will jump by £74 to £92 come 2019. All of a sudden this is no longer spare change."
Long went on to say that the government's models include assumptions from eight years ago, and that it is important that they reflect the Department for Work and Pensions' expectations.
"Underestimating the number of opt-outs would result in more money going into pensions and the government spending more on tax relief, the bill for which the Treasury would be forced to find from somewhere," he added.
In order to avoid the prospect of auto-enrollment unravelling, Hargreaves Lansdown suggest that the government implement measures to:
Improve financial literacy
Time personal allowance increases to coincide with contribution hikes
Delay further contribution increases to above 8%
Possibly remove the ability to opt-out of retirement saving
Include self-employed in auto-enrolment.
"Dropping out of saving for even short periods can seriously harm your retirement. Someone with average earnings who only opts-out between the ages 22 to 25, could reduce their pension pot by £20,000 when they reach retirement.
"Given the increase in minimum contributions next April, further thought should be given to how to minimise opt-outs. The government could also look at focusing on better member communication.
"An improved understanding of likely opt-out rates would help inform these decisions."