U.S. government employes would have to work an extra 10 years to get full retirement benefits and contribute a bigger portion of their salary to their retirement fund under proposals President Reagan will send to Congress later this month.

The president's budget, according to sources, will ask Congress to change the law that now lets federal workers with 30 years of service retire at age 55 on full benefits. Under Reagan's plan, employes would have to work until they were 65 to get full annuities.

Congress also will be asked to raise matching government and employe contributions to the civil service retirement fund, sources said.

Rep. Steny Hoyer (D-Md.) said he had heard about some of the proposals, and considers them "an outrage." Hoyer, who serves on the House Post Office and Civil Service Committee and whose district includes many federal workers, said the proposals "are a continuation of the assault on the benefits of federal employes."

Hoyer said he expects the changes will run into "heavy opposition" on Capitol Hill.

An Office of Personnel Management spokesman said he could not comment on the proposals.

But other sources said the retirement changes are part of a package of personnel "reforms" designed to bring federal pay and fringe benefit practices into line with those in the private sector.

Workers and agencies now each make contributions to the fund equal to 7 percent of an employe's gross salary. That would be raised to 9 percent of salary beginning next year, and to 11 percent starting in 1985.

The civil service fund now pays benefits to more than 1.3 million federal and postal retirees and their survivors, including about 63,000 people in metropolitan Washington. More than 2.8 million government workers, including 342,000 here, now make contributions to the civil service retirement fund.

The budget, which goes to Congress Jan. 31, will also propose a major change in the way federal pension benefits are calculated, sources said.

Currently benefits are based on length of service and are based on the employe's highest average salary over a consecutive three-year period.

The Reagan plan would base future annuities on the highest five-year average salary. That would reduce benefits for most federal workers.

All of the proposals would have to be approved by Congress, which is also expected to consider putting new federal workers under Social Security.

An OPM official said nearly half of all government workers retire by age 60, compared with about 7 percent of the nonfederal work force.

Under the proposed changes, reportedly approved at a budget meeting over the weekend, government employes could still retire at age 55, but their annuities would be reduced by 5 percent for each year they were under age 65. If that change becomes law, federal workers retiring in the future at age 55 would get about half of the annuity they could receive by retiring today at age 55