The sluggish economy is taking a toll on Americans’ confidence in their retirement savings, a new survey found.

Only a quarter of working Americans are “very confident” that they will have enough money for basic living expenses in retirement, down from 42 percent last year. And two-thirds say they believe they will need to work at least three years longer than they had planned, because of the economic environment.

The findings come from the Sun Life Financial “Unretirement Index” Survey. The survey was conducted by Knowledge Networks in September, using a probability-based sample of roughly 1,500 working adults. The margin of sampling error is 3 percent.

The Unretirement Index is scored on a scale of 0 to 100, with 100 reflecting highest confidence about retirement. The Index score dropped from near the middle of the range (44) in September 2010, toward the bottom third of the scale (36) in September 2011, down 18.2 percent. The survey is the fifth in a series that began in 2008,

The survey also found less faith in the durability of government benefits. Confidence in the future of Social Security benefits plunged to 9 percent today, from 22 percent in 2008. And confidence about Medicare benefits has also plummeted, to 8 percent today from from 20 percent in 2008.

How confident are you about your plans for retirement?