The retirement savings of millions of workers could be at risk due to "gaps" in the system of pension regulation, MPs have warned.
The Commons Work and Pensions Committee said the success of the automatic enrolment (AE) programme - which has seen 6.1 million people enrolled into workplace pensions - could be undermined by a collapse in one of the savings schemes.
In particular, it expressed concern about the development of "potentially unstable" master trusts which provide occupational pension schemes for multiple employers who are otherwise unconnected.
The Pensions Regulator initially encouraged employers to enter such schemes as they were considered to be best placed to meet the standards necessary for "good outcomes" for savers.
However, in evidence to the committee, the regulator expressed concern it was not able to exercise stronger regulation over them and that some of the smaller master trusts "may not be run by competent people".
Pensions minister Baroness Altmann told the committee she shared concerns that there was no protection for winding up a master trust which failed, "which is why I am so exercised about it".
The committee said the Government should now include legislation in the forthcoming Queen's Speech to give the regulator new powers to enforce minimum financial and governance standards and to protect the assets of members in the event of a trust winding up.
"Gaps in pension law and regulation have allowed potentially unstable master trusts on to the market," it said.
"Should one of these trusts collapse, there is a very real danger that ordinary scheme members could lose retirement savings. There is also a risk that faith in auto-enrolment as a whole will be undermined."
The committee also expressed concern that employees could be encouraged to opt out of auto-enrolment in order to invest in the new Lifetime Isa (Lisa), announced by Chancellor George Osborne in the Budget, leaving them worse off in retirement.
"Whatever the attractions of the Lisa, it must not be presented as a direct alternative to AE. Savings under AE carry an employer contribution, which will not be available in the Lisa," it said.
"Opting out of AE to save for retirement in a Lisa will leave people worse off."
Committee chairman Frank Field said: "Auto-enrolment has been a tremendous success that will ultimately see approximately nine million people newly saving, or saving more, in a pension.
"Crucially now we must do much more to ensure that people's savings are put in the best possible place and are secure."
A Department of Work and Pensions spokesman said: "We welcome the committee's recognition that automatic enrolment has been a tremendous success and is enabling millions of people, many for the first time, to save for their retirement through workplace pensions.
"We will consider the recommendations within the committee's report and will respond in due course."