Pension plan funding levels among large publicly held U.S. employers dropped to a record low in 2011 as tumbling interest rates fueled a rise in the value of plan liabilities and mediocre investment returns held down asset growth, according to a Milliman Inc. survey released Thursday.

Defined benefit plans offered by the 100 U.S. employers with the largest pension programs were funded 79.2% on average, down from 83.9% funded in 2010 and 81.7% in 2009. The lowest prior funding level was in 2008 when plans were funded 79.5% on average.

On average, the plans earned 5.9% on assets last year, down significantly from 12.8% in 2010 and 13.9% in 2009.

In all, the market value of pension plan assets increased by about $37 billion to about $1.246 trillion in 2011. But the value of plan liabilities leaped by about $133 billion to about $1.573 trillion.

That funding deficit of nearly $327 billion was the biggest in the 12-history of the survey, Milliman said.

Among surveyed employers, Detroit-based General Motors Co. had the largest defined benefit program as measured by assets. In 2011, the automaker had $108.9 billion in plan assets and its plans were 81.1% funded, down from 82.6% in 2010. However, GM's $25.4 billion difference between its 2011 plan assets and $134.3 billion in benefit liabilities was by far the greatest of any of the 100 employers in the study.

With $86.6 billion in plan assets, Armonk, N.Y.-based IBM Corp. had the second-largest pension program in 2011. Its plans were 89.3% funded last year, down slightly from 91.6% in 2010, according to the survey.

FPL Group Inc., a Juno, Fla.-based energy and utility company, had the highest funded ratio in 2011 at 147.1%, down from 162.1% in 2010. Atlanta-based Delta Air Lines Inc. had the lowest funded ratio: 40.4%, down from 47.1% in 2010.

In all, 10 employers in 2011 had plans that were at least 100% funded, up from eight in 2010.

On the other hand, 15 employers in 2011 sponsored pension plans that were less than 70% funded, compared with just seven in 2010.

Employer plan contributions declined slightly to $55.1 billion last year, down from $60.3 billion in 2010.

By comparison, Milliman said annual plan contributions averaged $39.1 billion over the prior five years.

In conducting the analysis, Seattle-based Milliman analyzed financial reports of publicly held companies sponsoring the 100 largest pension programs for which full-year data was available.