U.S. consumer sentiment slumped in December as Americans were rattled by ongoing negotiations to avert the tax hikes and spending cuts set to come into effect in the new year, data showed on Friday.

The Thomson Reuters/University of Michigan's final reading on the overall index on consumer sentiment tumbled to 72.9 from 82.7 in November, worse than forecasts for 74.7.

It was the lowest level since July and also came in under December's preliminary figure of 74.5.

Talks to avoid the so-called fiscal cliff were thrown into disarray on Thursday evening when Republican lawmakers failed to back an effort by House of Representatives Speaker John Boehner that was designed to extract concessions from President Barack Obama.

Economists say the economy could fall back into recession next year if the changes are allowed to go into full effect.

Record numbers of consumers spontaneously mentioned their concerns that no resolution would be reached before year-end, the survey said.

"Even if something is passed in the next week, unless it includes an extension of the payroll tax holiday, as well as no increase in income taxes except for the wealthy, consumers are likely to be disappointed," survey director Richard Curtin said in a statement.

Of those surveyed, 27 percent said they were concerned about higher taxes, topping the prior high of 26 percent seen in August 2011 in the wake of the drawn-out debt ceiling debate.

U.S. stocks as measured by the S&P 500 index were down about 1.0 percent in morning trading as hopes faded that a fiscal deal would be reached soon.

Consumers were also less upbeat about the economic outlook, with 35 percent expecting unemployment to rise during 2013, up from 19 percent in October. Only one-third expected an uninterrupted economic expansion over the next five years.

The barometer of current economic conditions slipped to 87.0 from November's 90.7, while the gauge of consumer expectations fell to 63.8 from 77.6.

The survey's one-year inflation expectation edged up to 3.2 percent from 3.1 percent, while the survey's five-to-10-year inflation outlook rose to 2.9 percent from 2.8 percent.