Employers will have to put tens of billions of dollars in extra contributions into their pension plans next year as plan assets have tumbled due to the equities market slump and liability values have surged due to low interest rates.

Pension plan funding levels, which were buoyed this year by gains in the equities markets, now are near all-time lows, hammered in recent months by the sharp reversal in the equities markets and the continuing fall in interest rates.

Two studies released last week affirm those developments.

As recently as June, defined benefit plans offered by the 100 U.S. employers with the largest pension plans were on average 87% funded, according to an analysis by Milliman Inc.

But by the end of last month, the average funding levels of those plans dropped to 72.8%, with the difference between plan assets and liabilities hitting $438.9 billion, a record one-month increase in the amount of unfunded liabilities in the 11 years Milliman has been tracking such changes.

Other surveys also show the rapid erosion of pension plan funding levels.

A Mercer L.L.C. analysis of pension plans sponsored by companies in the S&P 1500, also released last week, found that the average plan funding level sank to 72% as of Sept. 30, down from 79% at the end of August. It also is a major drop from this year's peak funding status, set in April, when plans had an average funding level of 88%.

That 72% average funding level was near the all-time low of 71%—set in August 2010—while the $512 billion difference in September between assets and liabilities was a record.

One implication of the plunge in funding levels is obvious: Employers will put more money in their pension plans.

“We definitely will see higher contributions over the next few years,” said Ari Jacobs, retirement solutions leader with Aon Hewitt Inc. in Norwalk, Conn.

“We are talking about record contributions,” said John Ehrhardt, a Milliman principal in New York.

“Things will get worse before they get better,” said Sheldon Gamzon, a principal with PricewaterhouseCoopers L.L.P. in New York.

Towers Watson & Co. has projected that employers—excluding a few categories, such as nonprofit organizations—would have to contribute $175 billion to their plans next year, up from $163 billion this year and $91 billion last year.

Unless there is a “significant rebound in the markets, the numbers for 2012 are going to be a lot higher,” said Mike Archer, a senior retirement consultant with Towers Watson in Parsippany, N.J.

Many companies, which have been building up cash during the past couple of years, will be better able to meet those bigger contribution demands compared with the Great Recession, experts say. But the impact of the higher contribution requirements will vary.

“It will be a significant burden for some, but for others, who have been sitting on a lot of cash, it will not,” said Jonathan Barry, a Mercer partner in the consultant's Boston office.

Still, the higher contribution requirements and the difficulty in predicting contributions—due to volatility in the equities markets—will lead some companies to freeze their plans, continuing a trend that began about seven years ago.

“I suspect it will drive some to freeze their plans,” said Heidi Rackley, a partner in Mercer's Seattle office.

Some employers, though, say they intend to stick with their plans.

Offering a defined benefit plan “continues to be part of our overall benefits package,” said Joseph Molloy, vp-benefits and employee services at North Shore-LIJ Health System in Lake Success, N.Y. Mr. Molloy said the health care system remains on target to fully fund its plan by 2015.

Yet another reaction, experts say, will be changes in investment strategies, such as putting more plan assets in bonds and less in equities, as well as exploring approaches in which pension plan risk is transferred to a third party, with the employer retaining the plan.

For example, this year, Hickory Springs Manufacturing Co. in Hickory, N.C., purchased a “portfolio buy-in” policy from a unit of Prudential Financial Inc. Under that policy, Hickory Springs transferred just less than $75 million in plan assets to Prudential, while Prudential is responsible for about $75 million in benefits earned by about 1,000 Hickory Springs retirees.

“I do think we will see more activity in that area,” Mercer's Mr. Barry said.

Experts also expect a push by employers for federal lawmakers to at least temporarily ease funding rules, which Congress tightened considerably under the 2006 Pension Protection Act.

Congress already has responded twice in recent years to calls for relief. In 2008, for example, Congress gave employers whose plans slipped below certain funding levels, more time to boost funding.

In 2010, lawmakers also provided relief. For example, under one approach, employers could amortize their obligations over 15 years, rather than the required seven years, for funding shortfalls for any two years between 2008 and 2011.

But so many conditions were attached to the relief, such as requiring employers to make extra plan contributions if they paid any employee more than $1 million in compensation, that only a small percentage of employers took advantage of the provision, PwC's Mr. Gamzon said.