The Professional Air Traffic Controllers Organization will announce today that it overwhelmingly has rejected a tentative 3 1/2-year contract with the Federal Aviation Administration, largely because the agreement failed to address the union's demands for a shorter work-week and a more liberal retirement plan.

Voting on the pact ended last night, with more than 90 percent of the union's nearly 15,000 members casting ballots against the agreement, according to union sources who were working from unofficial tabulations.

After the official count is announced today, PATCO President Robert E. Poli is expected to call Transportation Secretary Drew Lewis to request immediate resumption of negotiations, union and Reagan administration sources said yesterday.

"It would be in everybody's best interest if the talks are expedited," said David A. Trick, PATCO director of operations. Delay in resuming talks could lead to a renewal of strike threats. Failure to restart negotiations almost certainly would spark a controllers' strike -- in defiance of federal law -- that could hurt the nation's $100-million-a-day airline industry, Trick said.

Lewis previously has said he would be willing to return to the bargaining table. Technically, however, he has no power to bargain for much of anything. Only Congress has the authority to approve most wage and working condition changes for the controllers, as it does for most other federal workers, excluding employes of the U.S. Postal Service and the Tennessee Valley Authority.