General Motors is using its huge pension fund in a way it never intended.

It had planned  and put money aside  for a steady march of retirees over time. But instead, tens of thousands of blue-collar workers, most in their 40s and 50s, are all becoming eligible for retirement benefits now, as the company rapidly downsizes.

And even as its pension fund faces this giant bulge in payouts, G.M. is not putting any new money in  the company is not required to make any contributions to the fund until 2013.

The longer this goes on, the weaker the fund will be and the more uncertain its long-term viability.

For now, the pension payments to its younger “retirees,” part of a deal G.M. negotiated with the United Automobile Workers union in 2007, allow the company to drastically shrink its work force without having to come up with the cash to pay severance. The payments also relieve some of the burden on social service programs in the countless factory towns and counties around the country with large numbers of G.M.’s newly jobless.