Note: The headline of a print version of this story in the Contra Costa Times incorrectly said 138,000 members of the Contra Costa Health Plan would be affected by this change next year. The correct number is 1,100 members.

MARTINEZ -- An East Bay health insurance plan offered under Covered California elected to withdraw from the exchange in 2015 because it could not meet a federal regulation.

The Contra Costa Health Plan, which says its 1,100 new enrollees will remain covered through the end of the year, ran afoul of a recently clarified rule that insurers must offer the same plan inside or outside the exchange.

"We are deeply disappointed that we are going to have to exit for the next year under the exchange," Patricia Tanquary, CEO of the health plan, told the Covered California board of directors at its monthly meeting Thursday in Sacramento.

Tanquary said that offering the same version of the Contra Costa Health Plan both inside and outside the Covered California exchange is so costly to administer that it will lead to increased rates. The plan serves 138,000 mostly MediCal residents in Contra Costa County.

From Oct. 1 through April 15, about 1.4 million people in California enrolled in a health-care plan through the Affordable Care Act, also known as Obamacare, including 39,349 residents in Contra Costa County. Statewide, about 2 million also have signed up for Medi-Cal, the state's health care program for the very poor.

Statewide, the most common health insurance plans selected by enrollees were from the four largest insurers on the exchange: Anthem Blue Cross of California, Blue Shield of California, Health Net and Kaiser Permanente.

The other seven are local plans, and in the Bay Area include the Contra Costa Health Plan, the Valley Health Plan in Santa Clara County and the Chinese Community Health Plan in San Francisco and San Mateo counties. The Alameda Alliance for Health plan was offered to Alameda County residents on the exchange until Nov. 1, 2013, when it was removed by the state Department of Managed Health Care after financial problems surfaced.

Last month, the same department seized the Alameda Alliance, saying it was at risk of going under because it was not able to control its medical costs. A conservator was named to oversee the nonprofit public health care provider, which serves 204,000 mostly poor residents in Alameda County.

Marta Green, a department spokeswoman, on Thursday said Contra Costa Health Plan's case "has absolutely nothing to do with Alameda Alliance or the situation that that plan was facing."

Tanquary told the Covered California board that she hopes federal rules will be altered to allow the Contra Costa Health Plan to return to the exchange as early at 2016.

As for those currently on the Contra Costa Health Plan, Tanquary said, "We will personally call every single member to help them make changes to their plans for next year."

The next open enrollment period, for 2015 health care plans, starts on Nov. 15.

Contact Tracy Seipel at 408-920-5343. Follow her at Twitter.com/taseipel.