Now Pensions has overhauled its administration system after historic issues caused a failure in collecting and investing around 265,000 members' contributions.
The Pensions Regulator (TPR) has confirmed "serious and persistent administrative failings" had been addressed for the majority of affected members, which had led to around £18m of contributions not being collected.
The issues had occurred between 6 April 2015 and 8 August 2017, leading TPR to fine the master trust's trustees £50,000 and issue a further £20,000 penalty for failing to keep members properly informed.
All members have now been moved to a purpose-built platform, named ‘Gateway', while member data has been improved and data has been rebuilt for more than 350,000 members; all of the contributions for these members have now been collected and invested.
A small number of members are still owed contributions, however, but these cases largely relate to employer insolvency events. TPR said it was "continuing to monitor" the progress being made for these members, with Now Pensions "working proactively" for a resolution.
In a regulatory intervention report published today, TPR said there had been a "genuine change in the scheme's governance and administration", supported by changes in key personnel, and the trustee and Now Pensions "adopting a more proactive approach and taking responsibility for making improvements."
TPR executive director of frontline regulation Nicola Parish said the situation had been "unacceptable".
"When we launched our investigation into Now Pensions, the master trust had significant administration problems in the way it was handling data," she said. "In particular, its failure to collect contributions was causing problems for employers and the pension pots of members were not growing as they should have been."
She added: "Pension schemes, including master trusts, should be in no doubt that we will act if we become concerned about the way they are being run. We will not accept failings that put members' savings at risk."
Now Pensions Trust trustee chairman Nigel Waterson said: "The trustee directors and Now Pensions have worked closely with TPR throughout this process and we are very pleased with today's report which recognises we are largely back to business as usual.
"Feedback we've had from those using the Gateway system has been hugely positive and with the significant investment and improvements that have been made throughout the business, Now Pensions is set to go from strength to strength."
The master trust's chief executive, Troy Clutterbuck, apologised for the "length of time it's taken to put things right".
The case concludes as master trusts are being required to apply for authorisation by the end of March in order to continue operating in this space. Over a third of master trusts have already confirmed they will exit the market, with their members and assets transferred to another authorised vehicle.
Waterson confirmed the master trust was working on its application and that it would be submitted "in due course".
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More than 4,500 retail investors affected
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Paid out £54m in related compensation