By Caroline Humer

REUTERS - As millions of people shop online for new insurance plans under President Barack Obama's healthcare reform law, about 10 percent of applications to the main website are not being accurately transmitted, a government spokeswoman said on Friday.

The error rate has improved since HealthCare.gov's disastrous debut on October 1, which created a political crisis for Obama as opponents and supporters of the law questioned whether the most sweeping social program since the 1960s was rushed well before it was ready.

Last week, the White House wrapped up a five-week emergency effort to fix the most obvious of the website's technical problems and between midnight on Sunday and noon on Friday, government health officials said 3.7 million people visited.

But the current level of mistakes deeper into the system are creating a new workload for insurers to verify the details of their new members and could lead to unwelcome surprises for consumers if there are mistakes in new policies that take effect on January 1.

HealthCare.gov relays information about new customers in so-called "834" transaction forms to the private insurance companies that provide the health plans. The portal serves people in 36 states while 14 states are running their own online marketplaces.

"We believe nine of 10 transactions are being successfully transmitted," the Centers for Medicare and Medicaid Services, or CMS, spokeswoman Julie Bataille said at a news briefing.

Previously, Bataille has said the majority of errors had been fixed in these transactions but had not defined either the current or past rate of mistakes.

She said the error rate has fallen, explaining that in the more than two months since October 1, HealthCare.gov could have experienced an error in as many as one in four transactions. Errors include forms not being generated, transmitting duplicate forms and of forms with technical data being passed incorrectly, she said.

Widespread errors in coverage could add to public complaints over the law's rollout and provide ammunition to Republican lawmakers who have campaigned all along to delay or replace the 2010 Affordable Care Act, commonly known as Obamacare.

"The new process put in place this week is making a difference. The enrollment files are getting better, but there is more work to do to ensure consumers are covered," Karen Ignani, the chief executive officer of insurance industry trade group, America's Health Insurance Plans (AHIP), said in a statement on Friday.

BACK-UP SYSTEM

Insurers are also working with CMS on testing a short-term back-up system for the government to pay insurers for the income-based subsidies that many individuals will qualify for on the new online exchanges. The permanent system was not built before the website opened for enrollment.

The glitch-ridden consumer end of the website held back enrollment during October, when fewer than 27,000 people enrolled through HealthCare.gov. The government set a November 30 deadline for repairs. On Sunday and Monday alone, 29,000 people signed up, sources familiar with the number said.

The enrollment growth has raised questions about how insurers will keep up if the applications have errors and whether consumers will think they are covered by an insurance policy when they are not.

For instance, Jason Alford, the director of individual and marketplace sales at Health First Health Plans in Rockledge, Florida, said in an interview this week that applications are still incorrectly displaying if the person is a smoker or a non-smoker, a factor that determines the premium price in the state.

CMS said that there is a technology team dedicated to fixing the issues with the transaction forms. The team is overseen by the general contractor on the project QSSI, a unit of UnitedHealth Group Inc (NYS:UNH).

In an unusual show of solidarity, AHIP, CMS and the Blue Cross Blue Shield Association - whose licensees are the largest players on the insurance exchanges - this week issued a joint statement to stress their efforts to fix "back end" problems on HealthCare.gov.

Daniel Durham, vice president for policy and regulatory affairs at AHIP, said this week that insurers will estimate how much they are owed, and submit that estimate to the government.

Once the permanent system is built, the government and insurers can reconcile the payments made with the plan data to "true up" payments, he said.

(Reporting by Caroline Humer in New York, David Morgan and Roberta Rampton in Washington; Editing by Michele Gershberg and Grant McCool)