WASHINGTON (Reuters) - Kathleen Sebelius, the U.S. health secretary who oversaw the botched rollout of President Barack Obama's signature healthcare reforms, has resigned, a senior administration official said on Thursday.

Obama has chosen Sylvia Mathews Burwell, his budget director, to replace Sebelius, a second official said. Obama was due to announce the change with Sebelius and Burwell at his side at a White House event at 10:45 ET on Friday.

Sebelius, 65, became the public face for the problem-plagued start to the enrollment period for Obamacare, which was meant to reduce the number of Americans without health insurance and cut into massive U.S. healthcare costs.

When enrollment opened in October, a website used to shop for insurance in 35 states, HealthCare.gov, failed to work for weeks, and became the lightning rod for criticism from Republicans, who want to repeal the program.

The enrollment was ultimately successful, surpassing the seven million figure the Obama administration had predicted, but Sebelius, a former governor of Kansas, told Obama in early March she wanted to leave the administration.

"She believed that once open enrollment ended it would be the right time to transition the department to new leadership," an official said.

Burwell, a former official at the Gates Foundation and Wal-Mart Foundation, helped the administration manage its response to a shutdown of the federal government brought on by a budget battle with Republicans in October. She also was a key player in talks that yielded a two-year budget agreement in December.

"The president sought a nominee with strong credentials in management, implementation, and performance for this important role," the official said, noting she was confirmed unanimously to lead the budget office less than a year ago.

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