The FBI will assume a major role in drug law enforcement under a reorganization in which the Drug Enforcement Administration will begin reporting to the FBI, the Justice Department announced yesterday.

The plan does not call for a complete merger of the two agencies, both of which will have authority to investigate drug law violations. Up to now the FBI has had jurisdiction only when drug cases developed from organized crime investigations.

President Reagan is expected to nominate Francis M. Mullen Jr., one of three executive assistant directors of the FBI, to head the DEA. Mullen has been acting administrator of the DEA since July.

Attorney General William French Smith said, "For the first time in our nation's history" the reorganization will "bring the full resources of the FBI to bear on the problem of domestic drug trafficking. I am confident that an infusion of FBI resources to supplement those of DEA will aid immeasurably in our nation's drug enforcement effort."

Initially, the FBI will help on drug-trafficking cases that are linked to organized crime. Mullen said about 25 percent of the FBI's organized crime investigations involve drug trafficking.

Smith would not say whether the reorganization will require additional funding from Congress.

"We believe this combination will produce efficiencies in achievement of the ultimate results which in the long run will save resources," he said.

But FBI Director William H. Webster said, "I can't tell you in candor that efforts we make in drugs will not require reallocation from other efforts."

Up to now, the DEA has operated as an independent unit within the Justice Department, reporting to the attorney general. The change will mean that the DEA will report to Webster, who reports to the attorney general.

Since Mullen became acting head of the DEA in July, the number of joint DEA-FBI investigations has grown from 15 to 125. Smith said the reorganization will involve rotating agents between the agencies.

With the FBI's help, Smith said, the DEA will be able to do more court-authorized wiretapping than it has been able to do with limited staff.

Smith said it was possible that the two agencies might merge in the future, but conceded that there are problems with a merger now. One of the biggest is the difference in hiring qualifications and training between FBI agents and DEA agents.

FBI agents must be college graduates. They are also in a special federal employe category called "excepted service," and can be hired and fired by the FBI director without the protection of normal civil service procedures. DEA agents are civil service employes who don't have to be college graduates.

Smith is recommending that the differences be eliminated by administrative and congressional action.