WASHINGTON (Reuters) - The U.S. government asked the Supreme Court on Friday not to allow Roman Catholic-affiliated groups a temporary exemption from a part of the Obamacare healthcare law that requires employers to provide insurance policies covering contraception.

On Tuesday night, Justice Sonia Sotomayor had granted a temporary injunction preventing the government from enforcing the so-called "contraception mandate" against the Little Sisters of the Poor and Christian Brothers Services while litigation continues.

In its filing on Friday, the government said the groups have no legal basis to file the lawsuit because the insurance plan in question is a "church plan," which the government has already acknowledged it cannot force to provide contraception coverage.

Little Sisters of the Poor is a Baltimore-based order of nuns that runs nursing homes across the country. Christian Brothers Services administers healthcare plans for Catholic organizations.

Now that the court has received the government's filing, it will decide whether to extend the injunction while the case continues in lower courts. There is no deadline for court action, and Sotomayor can make the decision herself or refer it to the whole court, in which case all nine justices will decide.

The case coincides with the expansion this year of coverage under the 2010 Obamacare law, the most sweeping U.S. social legislation in 50 years.

The religious organizations accuse the federal government of forcing them to support contraception and sterilization in violation of their religious beliefs or face steep fines.

'PERMISSION SLIP'

The 2010 Affordable Care Act, known as Obamacare, aims to dramatically reduce the number of Americans with no health insurance, estimated by the government at more than 45 million.

Republicans oppose the law, and hope their stance gains them seats in November's Congressional elections, especially after glitches with the website used to enroll people in coverage.

The healthcare law requires employers to provide insurance covering preventive services for women, including contraception and sterilization. There is an exception for religious institutions such as houses of worship that mainly serve and employ members of their own faith, but not schools, hospitals and charitable organizations employing people of all faiths.

As a compromise, the administration agreed to an accommodation for non-profits affiliated with religious entities that was finalized in July.

Under the accommodation, eligible non-profits must provide a "self certification", described by one lower court judge as a "permission slip" authorizing insurance companies to provide the coverage. Challengers say that step alone violates their religious rights, regardless of whether employees ultimately receive contraception coverage.

They also point to court papers in which the government has said it is still looking at ways to enforce the contraception mandate against church plan administrators.

In a separate case in U.S. District Court in Washington, D.C., the government said in a November filing that officials "continue to consider potential options to fully and appropriately extend the consumer protections provided by the preventive services coverage regulations to self-insured church plans."

A federal judge in Colorado, William Martinez, denied the plaintiffs' request for an injunction on Dec. 27. The Denver-based 10th U.S. Circuit Court of Appeals followed suit on Dec 31, prompting the last minute plea to the Supreme Court.

Throughout the country, Catholic groups are asking the courts to exempt them temporarily from the mandate while litigation continues. Most courts have granted the requests. The mandate, which was due to take effect for the organizations on Wednesday, is already in place for many women who have private health insurance.

In separate cases, the Supreme Court already has agreed to hear oral arguments on whether for-profit corporations have the basis to object to the contraception mandate on religious grounds. The court is due to hear the arguments in March and decide the two consolidated cases by the end of June.

(Editing by Howard Goller, Chizu Nomiyama and David Gregorio)