(Adds details of report and quotes from hearing)

By Roberta Rampton and Caren Bohan

WASHINGTON, Nov 19 (Reuters) - President Barack Obama, who has portrayed himself as surprised by technical problems with the government's new health care website, was briefed earlier this year on a consultant's report that warned of possible widespread site failures, the White House said on Tuesday.

There have been weeks of questions about whether Obama understood the depth of the site's problems and let it open anyway, or simply "did not have enough awareness" of them, as the president stated at a Nov. 14 news conference.

While the government says it is improving the portal's performance every day, security experts told a Republican sponsored congressional hearing Tuesday that in their opinions, it is still not sufficiently secure to be used confidently by consumers.

Even as the administration fended off criticism of the so-called "front end" of the system, officials revealed Tuesday that they had not completed development of the "back end," the financial management component needed to finalize federal subsidies for consumers who buy health plans.

A spokeswoman for the Centers for Medicare and Medicaid Services, the lead agency for the website, said it would not be completed until mid-January, weeks after the first enrollees are scheduled to begin receiving benefits under the Affordable Care Act, passed in 2010 as Obama's signature domestic policy.

The law, commonly called Obamacare, mandated that Americans have health insurance and created new online marketplaces to buy and sell policies.

Meanwhile, Obama's approval rating dipped to a low of 37 percent in a Reuters/Ipsos poll.

Bits and pieces have leaked out over the past few weeks about flaws in the site's development process. Monday night, however, Republican lawmakers who oppose Obamacare released a report and recommendations prepared by McKinsey & Co at the government's request in March 2013.

It cited, among other things, a rushed process that left insufficient time for testing and a focus by officials on getting people enrolled versus making the system work right.

The consequence, it said, could be system failures that could make enrollment slow or at times impossible for consumers, which is exactly what happened.

Questioned about the McKinsey study, White House spokesman Jay Carney said the president had been briefed on it in the spring.

But he said the president's familiarity with the report and recommendations did not contradict previous statements from the White House that described Obama as surprised by the scope of flaws in HealthCare.gov.