Henry Loomis, president of the Corporation for Public Broadcasting since October 1972, has announced his intention to retire no later than September of next year.

Although that is the date of the annual CPB board meeting at which contracts are routinely renewed, Loomis said he is "prepared to step down as soon as the new chief executive has been elected and assumed leadership of the corporation."

The departure of Loomis, who became president at the height of the Nixon administration's efforts to control public broadcasting, has been rumored since the Carter administration took office last January.

The search for his successor could begin as early as next month, when the current 15-member CPB board meets in Columbia, S.C.

However, President Carter will be able to choose five new members of the board after March, when five incumbents' terms expire, and a serious effort to find Loomis' successor is not expected until spring.

That timetable would fit in with current plans on Capitol Hill to begin consideration of the administration's new public broadcasting financing bill by late February. Among other proposals, the bill includes a restructuring of CPB's role in public broadcasting.

The departure of Loomis, whose tenure rightly or wrongly has been closely identified with the Nixon attempt to "politicize" CPB, is expected to facilitate passage of the Carter proposal.

A White House spokesman yesterday was careful to point out that the administration "won't have anything to do with selection of a CPB president except to the extent the new president will be hired by the board dominated by Mr. Carter's appontments."

Already serving on the CPB board as Carter appointees are Sharon Percy Rockefeller and Gillian Sorenson. Confirmation of a third Carter nominee. Irby Turner Jr. of Belzoni, Miss. has been held up in the Senate pending investigation of changes of discrimination recently made by various rights groups.

Those three and the five due in the spring would give Carter nominees control of the 15-member board.

Barry Jagoda, special assistant to the President, said yesterday that "we are gratified that Henry has agreed to stay on until the new board can choose a successor and it seems appropriate that he will provide transition leadership during this period.

"While Henry Loomis has provided distinguished leadership service," Jagoda said, "not only at CPB but throughout government over the last two decades, as 1978 begins we are looking at the possibility of a significant leap forward in public broadcasting."

Jagoda said, "We plan to stay clear of the selection process but we will follow it closely."

He said the one requirement the White House hopes that selection committee keeps in mind is that "whoever is chosen will protect public broadcasting from unwarranted political interference over the years."

That point was made repeatedly last fall in the presidential message accompany the financing bill to the Hill.

The 58-year-old Loomis, a member of a distinguished Eastern family, began his government service with the Department of Defense in 1950 and subsequently served under five Presidents.

Although Loomis, who is independently wealthy, considers himself a non-political public servant in the British mold, his tenure at CPB began as the Nixon-appointed board of directors made what critics saw as a series of decisions designed to sharply curtail innovation and public affairs programming that had displeased the Nixon administration.

In an exchange of letters last week with CPB board chairman W. Allen Wallis, chancellor of the University of Rochester. Loomis said, "It is imperative that the president of the corporation be changed in an orderly manner." The letters were released yesterday.

He told Wallis, "I have worked in the pressure-cooker atmosphere of Washington for 25 years. My five years with the corporation have been both challenging and rewarding but I have long looked forward to a more normal life with my family."

Loomis was on the West Coast yesterday and unavailable to comment further.

Loomis and his second wife, the former Jacqueline C. Williams, maintain a farm in Middleburg, Va.

While it is too early to speculate on a possible replacement at CPB, the $70,000-a-year post is bound to attract many applications.

One name that has been rumored in the past is that of John Ganz Cooney, the founder of Childrens Television Workshop, whose "Sesame Street" and other programming has been a staple of public TV over the years.