WASHINGTON (Reuters) - About 20 percent of the U.S. population delayed or were unable to get access to medical care when they needed it in 2007, up from 14 percent four years earlier, a study released on Thursday found.

About 9.5 million more people went without medical care in 2007, compared with 2003, the nationally representative survey released by the Center for Studying Health System Change, a nonpartisan policy group, found.

In a striking finding, the survey said although those without insurance were more likely to report going without care, those with insurance had a greater percentage increase in unmet medical needs.

“It’s not a pretty picture, especially for insured people, who are increasingly finding that the access to care once guaranteed by insurance is declining,” said Peter Cunningham, co-author of the study, which was funded in part by the Robert Wood Johnson Foundation, which provides grants for projects aimed at improving U.S. health care.

The telephone survey of 18,000 people was based on a random national sample with 43 percent of those polled responding.

Cost was the biggest obstacle to care for both the insured and the uninsured, the study said. For the insured, individuals said they were unable to get their health insurer to pay for treatment, or that a doctor or hospital would not accept their insurance.

About 47 million people in the United States do not have health insurance, a number that has been climbing since 2000.

At the same time, medical costs - driven by drugs, hospital and doctor fees -- have risen at least twice the rate of inflation for several years, making it more expensive for those with insurance to afford care.

UnitedHealth Group, WellPoint Inc. and Aetna Inc. are the three biggest U.S. health insurers.