By Jeff Mason

WASHINGTON (Reuters) - Chris Jennings, a top adviser to President Barack Obama who played a key role in the implementation of his signature healthcare law, is leaving the White House for health and family reasons, administration officials said on Thursday.

Jennings, a respected healthcare policy adviser in the Clinton administration, was brought on board last summer before the flawed rollout of the Affordable Care Act, popularly known as Obamacare, created one of the biggest political headaches Obama has faced since entering office.

"Chris served the country at a time when he was needed most," White House Chief of Staff Denis McDonough said in a statement.

"He will be deeply missed, but we all wish him the best and know that he will continue to be a key player in health care and always a champion for quality affordable health care for all Americans."

An aide who spoke on condition of anonymity blamed a recent health scare and "other serious family considerations" for Jennings' departure.

Jennings has more than 30 years of experience in health policy and served as a senior adviser to President Bill Clinton for several years.

His arrival at the Obama White House was announced in July ahead of what proved to be a critical juncture for the law. The rollout of the program's primary website, HealthCare.gov, became a disaster as deep technical problems prevented people from signing up for coverage.

The website has improved since then, and the administration has brought in other officials to fix the implementation problems, including former legislative director Phil Schiliro and John Podesta, a onetime chief of staff to Clinton who now serves as counselor to Obama.

Associates of Jennings inside and outside the White House played down the impact of his exit.

"It would concern me more if there were a whole raft of departures. You're not seeing that," said Dan Mendelson, chief executive of consultancy Avalere Health and a former colleague of Jennings in the Clinton administration.

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"He helped them a lot before he went into the White House, and I'm sure he will help them a lot after he leaves."

The aide who spoke on condition of anonymity said Jeanne Lambrew, another top healthcare adviser, and the rest of the team would continue to focus on bringing healthcare costs down and implementing the program in Jennings' absence.

"It has been a great privilege to work with the president and his incredibly dedicated team on the Affordable Care Act," Jennings said in a statement.

"This is the cause of my professional life and I look forward to making continued contributions to that end."

(Editing by Jonathan Oatis)