The National Employment Savings Trust (NEST) will not break even until 2026, at which point its debt to the Department for Work and Pensions (DWP) will have hit £1.2bn.
The loan will then not be paid off until 2038, a quarter of a century after the government-backed master trust was set up, according to government predictions.
The debt totalled £539m at the end of March but is now expected to more than double in the next nine years. However, post-2026 membership and contribution levels are expected to reach a point where income from annual management charges and contribution charges - which sit at 0.3% and 1.8% presently - will overtake loan repayments.
The estimates were revealed in a letter from DWP permanent secretary Sir Robert Devereux to Public Accounts Committee chairwoman Meg Hillier.
However, Sir Robert added that "there are a number of uncertainties in forecasting the evolution of the loan balance, and its eventual repayment".
These include membership growth, contribution levels, funding growth, interest rates, and earning assumptions not meeting expectation, which could all lengthen the repayment schedule.
It also referenced market uncertainty, macro-economic uncertainty, and member behaviour.
The DWP also noted recent policy changes, such as the removal of the annual contribution limit and transfer restrictions, could impact NEST's revenue and therefore delay repayment.
At the end of March, NEST had 2.7 million active members, 370,000 participating employers and more than £1.6bn of assets under management. These are expected to continue growing as the roll-out of auto-enrolment (AE) concludes.
The success of AE meant "NEST's finances have now moved from a position of uncertainty to one where there is more stability in the forecast", Sir Robert added.
NEST chief executive officer Helen Dean said the master trust repayment schedule remained on target.
"NEST was set up so AE worked for the mass market, ensuring every employer had access to a high quality provider and every worker, including lower earners, had a great scheme to save in," she said. "We're doing the job we were set up for and we're on track to become self-financing well within the original range forecasted by government and the European Commission."
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