Secretary of Education Arne Duncan has announced that he will unilaterally override the centerpiece requirement of the No Child Left Behind school accountability law, that 100 percent of students be proficient in math and reading by 2014.

Mr. Duncan told reporters that he was acting because Congress had failed to rewrite the Bush-era law, which he called a “slow-motion train wreck.” He is waiving the law’s proficiency requirements for states that have adopted their own testing and accountability programs and are making other strides toward better schools, he said.

The administration’s plan amounts to the most sweeping use of executive authority to rewrite federal education law since Washington expanded its involvement in education in the 1960s.

Conservatives said it could inflame relations with Republicans in the House who want to reduce, not expand, the federal footprint in education. But Mr. Duncan and White House officials described their plan as offering crucial relief to state and local educators as the No Child law, which President George W. Bush signed in 2002, comes into increasing conflict with more recent efforts to raise academic standards.