"The longer the delay in making the changes, the more disruptive and painful they will be for participants, plans, and employers," according to the report.

PBGC's projections show that the agency's reserve funds are all but guaranteed to be depleted by 2036.

Whenever the program does become insolvent, participants in failed multiemployer plans will see their insured benefits cut. PBGC will only be able to pay benefits from the revenue it receives from annual premium payments. PBGC collected $300 million in premium payments from multiemployer plans in 2017.

The multiemployer program claims $2.2 billion in total assets, against more than $67 billion in future liabilities.

The agency booked $53 million in investment losses in FY 2017, only the second time multiemployer assets lost value since 2008. In 2013, investment losses were $96 million. PBGC is required to invest multiemployer premiums in U.S. Treasuries, according to the report.

Other revenue, such as pension assets PBGC claims when it takes over a pension, can be invested in equities. Of the multiemployer program's $2.2 billion in assets, about $1.9 billion is invested in U.S. Treasuries.

Administrative expenses for the program were $42 million in 2017, also the highest over the past decade.

To date, 72 multiemployer plans are insolvent, accounting for $2.6 billion of the program's overall deficit.

But the brunt of the agency's deficit comes from the 47 plans that are projected to go insolvent in the next 10 years. Those plans account for more than $62.5 billion of the deficit, the report says.

"In the coming years, the demand for financial assistance from PBGC will increase as more and larger multiemployer plans run out of money and need help to provide benefits at the guarantee level set by law," according to a statement from PBGC. "The assets and income of PBGC's Multiemployer Program are only a small fraction of the amounts PBGC will need to support the guaranteed benefits of participants in plans expected to become insolvent during the next decade."

The $141 million in financial assistance paid in 2017 was the most assistance retirees received over the past decade.

The multiemployer program covers about 10.6 million participants in about 1,400 plans. About 10 percent of covered workers are in plans projected to go insolvent.

The flat rate premium plans pay increased from $27 per participant in 2016 to $28 for 2017. PBGC does not have the statutory authority to set premiums rates. That power resides with Congress.

Most of the country's union workers' pensions are insured by PBGC. But those pensions are protected at substantially lower levels than the private-sector single employer pensions insured by the agency.

For instance, a retiree with 35 years of service is only guaranteed about $15,000 in pension payments after a plan goes insolvent. A retiree with 20 years of service  will only get about $8,500 in PBGC benefits if their plan goes insolvent.