The Reagan administration will send warning letters to the 183 young men whom Selective Service has reported to the Justice Department for failing to register for the draft, the White House said yesterday.

David Gergen, White House communications director, added that President Reagan is still considering whether to end compulsory registration, which started under President Carter. Reagan opposed compulsory registration during his election campaign.

Selective Service director Thomas K. Turnage told The Washington Post on Monday that 800,000 18-year-old men have failed to register, including about 300,000 born in 1963--the group that is supposed to register this year. Turnage said that "we know of no significant major organization that is against registration."

Senate Appropriations Committee Chairman Mark Hatfield (R-Ore.), who opposes compulsory registration, issued a statement yesterday declaring that "I am shocked that director Turnage is marching so seriously out of step with the president."

Hatfield said one of the "significant" organizations that has opposed compulsory registration "is an organization known as the Office of the President. It is headed by a man who stated to me in writing that the most fundamental objection to draft registration in a free society is 'moral.'"

In another congressional reaction to the failure of 800,000 young men to register, Rep. Martin O. Sabo (D-Minn.) said yesterday the Selective Service picture is even worse because "700,000 who did register already have outdated addresses in the government's computer."

"The large number of obsolete addresses, when combined with the large number of men who have not signed up at all, shows that peacetime registration currently fails to meet its designed goal of a rapid and fair mobilization in a time of national emergency," said Sabo, a member of the House Appropriations subcommittee that handles the Selective Service budget.