The Obama administration went to court Thursday to block two major health insurance mergers, siding with consumer advocates and medical groups worried that the consolidation of large national health plans could lead to higher premiums.

The long-anticipated move by the Justice Department and attorneys general in California and 10 other states,  will at least temporarily prevent Anthem Inc.’s $48-billion purchase of Cigna Corp., a combination that would create the nation’s largest health insurer.

And it will stop Aetna Inc.’s $34-billion bid to acquire Humana Inc., a merger that would have combined the nation’s third- and fifth-biggest health plans.

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