Attorney General William French Smith did not oppose but was the originator of the Reagan administration plan to add a Cabinet council on legal affairs to the five existing councils that coordinate other areas of policy, Justice Department spokesman Thomas P. DeCair said yesterday.

The Washington Post in Sunday editions reported that Smith initially opposed adding a Cabinet council on legal affairs, which the president has decided to set up to thrash out policy on such issues as the Equal Rights Amendment and tax exemptions for private schools, on both of which the administration has recently been embarrassed.

DeCair said the source of the Post story must have been someone "who is either completely uninformed or who is pushing some ulterior motive with intentionally inaccurate information." He said that explained why the Post sources had asked to be anonymous.

Smith, according to his spokesman, proposed adding a legal affairs council last month. Deputy White House press secretary Larry Speakes said yesterday that presidential counselor Edwin Meese III also indicated that Smith was the one who proposed the establishment of the council.

Other sources indicated that a legal affairs coordinating group had been under consideration even before December.

Asked what had happened to the proposal since December, DeCair said that he did not know. The Post story "was the first time that we knew it had been decided to go ahead with the council," he said.

The council, which will be chaired by Smith, will join the other councils--on economic affairs, commerce and trade, natural resources and environment, food and agriculture, and human resources--which President Reagan established Feb. 26.

The councils are designed to be forums for the coordination of policy on issues that involve several government departments and agencies.

The administration had to do embarrassing about-faces this month on both the tax exemption and the ERA issues. The Justice Department had a leading role in policy formulation on both issues.