The House Judiciary Committee yesterday began work on a broad revision of immigration laws which would grant amnesty to millions of aliens who are already in this country illegally, but impose penalties on employers who hire such persons in the future.

Sponsors say the controversial legislation is essential if the United States is to regain control of its borders. An estimated 3 million to 6 million illegal aliens live here now, and up to a million more come in each year, particularly over the Mexican border.

However, even if the committee finishes marking up the bill this week, it is unclear whether it will reach the floor as Congress rushes toward its scheduled early October adjournment. The Senate took a week with 20 hours of debate and 31 roll-call votes to pass a similar bill last month.

In a testy letter to Judiciary Chairman Peter W. Rodino Jr. (D-N.J.), House Majority Leader James C. Wright Jr. (D-Tex.) said Congress should not try to "rush" the bill to passage and said he had "serious concerns" about the amnesty and other provisions.

Amnesty, he suggested, would be more supportable if limited to persons who could show they had steady work histories and had done no serious lawbreaking, adding that "we might also expect on the part of those who seek permanent amnesty a showing that a serious attempt is being made to learn English and, even more important, to insist that their children do so."

Meanwhile, the Mexican-American Legal Defense and Educational Fund, a private advocacy group, charged from the opposite direction that "job discrimination, invasion of privacy and shattered families may face Hispanics nationwide," if the bill passes. The group contends that the employer sanctions will end up causing discrimination against Hispanics.

However, Rodino called employer sanctions "the cornerstone" of the bill and said critics would legitimize "the status quo of an underground, undocumented, unrepresented and vulnerable population."

The bill, which would be the first major revision of immigration law in 30 years, would penalize employers who knowingly hire undocumented aliens. A companion provision calling for a fraud-proof identification system has led civil libertarians to protest that it could lead to a national identification card.

Besides the provisions dealing with illegal immigration, the bill places a cap of 450,000 a year on legal immigration, which does not include refugees such as Southeast Asians or Ethiopians admitted for political or humanitarian reasons. In 1980, the United States admitted more than 800,000 immigrants, including the unusual group of 135,000 Cuban and Haitian "boat people."

Efforts are expected to attach a guestworker provision to the bill, allowing temporary Mexican workers to enter the country more easily than they can under present program which requires certification that the employer cannot find suitable Americans to do the job. Over the last decade, an average of 18,300 workers a year have been certified, particularly for farm work.