The Obama administration is again extending a temporary Obamacare program insuring some of the sickest patients in the country amid concerns not enough people will find new coverage by the end of the month.

People enrolled in the temporary Pre-Existing Condition Insurance Plan, which was set to expire March 31, can purchase an extra month of coverage, the Obama administration said this afternoon. The PCIP program covers people who traditionally had trouble affording or finding individual insurance because of a medical condition.

The White House is standing by the March 31 deadline for enrollment, but the administration's announcement shows that officials are carving out more exceptions to it.

“As part of our continuing effort to help smooth consumers’ transition into Marketplace coverage, we are allowing those covered by PCIP additional time to shop for new coverage while they receive the ongoing care and treatment they need," Centers for Medicare and Medicaid Services spokesman Aaron Albright said in a statement.

The extension is available to PCIP enrollees who haven't yet enrolled in another health plan. They have until April 15 to enroll in new coverage that starts May 1.

The administration said it wanted to avoid a possible lapse in coverage for PCIP patients, which include many people getting treatment for diseases like cancer and diabetes. About 21,000 people are still in the program, according to enrollment figures updated today.

This marks the third extension of the Affordable Care Act’s $5 billion high-risk pool program, which under the health care law was supposed to sunset at the end of 2013. In mid-December, CMS extended the program until January. A month later, CMS said it would keep the high-risk pools open through March 31.

The ACA program, launched in 2010, was meant to serve as transition to 2014, when insurers can no longer deny people coverage based on a pre-existing condition or charge them more for it.

Enrollment in the program was lower than expected, but the administration struggled with low funding. CMS suspended new enrollment in February 2013 in order to pay for existing members. About 100,000 people were enrolled in PCIP plans at the time.

Cancer advocates cheered Friday's extension.

“We’re pleased cancer patients and survivors with health coverage through the Pre-Existing Condition Insurance Plan can keep it for another month, so they can avoid a gap in coverage," the American Cancer Society Cancer Action Network said in a statement. "We encourage patients in PCIP to look at other insurance options, including the health insurance marketplace at HealthCare.gov, as soon as possible so they are assured of coverage beyond the short term.”

In a separate announcement Friday, HHS said insurers selling health plans in Obamacare exchanges cannot deny coverage to same-sex couples.