Director of Central Intelligence, Stansfield Turner, yesterday ended several days of speculation by telling a joint House subcommittee that he stands by his recent statement that the proposed sale to Iran of an advanced flying radar system could cost this country a vital edge in electronic technology.

Turner, in a letter to the General accounting Office that became public 10 days ago, said the sale of seven Airborne Warning and Control Systems to Iran could cost the nation military secrets. Since his letter became public, Turner has been under heavy pressure from other administration officials to change his assesment of the security risks.

Although he had not taken a public stand favoring or opposing the sale, Turner is the only administration official who had publicly raised questions about it.

Turner's testimony yesterday came at a closed session of the House Armed Services subcommittees on Europe and Middle East and a Science and Technology, which are considering a resolution to block the proposed sale.

Follwoing more than four hours of testmony from administration witnesses in favor of the sale and from Turner, the subcommittees decided to put the resolution before the full Armed Services Committee next Thursday, when Secretary of State Cyrus R. Vance and Defense Secretary Harold Brown are expected to urge the committee to defeat it.

A number of issues has been raised by President Carter's decision to sell Iran seven AWACS is basically a modified Boeing 707 jet carrying the latest in radar, jamming, and communications equipment, built by Westinghouse.

Among the key issues are the potential risks of having the technology fall into the hands of the Soviet Union, which shares a 1,200-mile border with Iran, and the sale's apparent contradiction of the arms export control policy Carter announced only two months ago.

In the letter to the GAO, Turner said that although Iranian security has proved equal to the task of guarding such sensitive weapons as the F-14 fighter and its Phoenix missile, AWACS might be so tempting that the Soviet Union - which experts say is 10 years from developing a comparable system -w ould make extra efforts to obtain it.

His letter became part of a GAO report that strongly criticized the sale on grounds of the potential security risk and of Iran's need for and ability to handle so complex a system.

Turner "stuck unequivocally by his earlier statements. He didn't change one word of what he said before," according to Rep. Gerry E. Studds (D-Mass).

Turner and the administration officials will testify today behind closed doors before the Senate Foreign Relations Committee, which also is considering a resolution to bar the sale. Both the Senate and the House must pass their resolution by Aug. 5 to prevent the sale from going through.