President Reagan's oldest son, Michael, yesterday said he would quit both his jobs as a result of the controversy surrounding letters he wrote, in which he used his father's name, soliciting business from several U.S. military bases.

"It's just so silly," the younger Reagan said. "Somebody else can write a letter to the military bases or to anybody and say "Hey, I think Ronald Reagan's a great president.' I write a letter and say my dad's a great president and I have the press on my doorstep."

The letters were written in Reagan's capacity as vice president of Dana Ingalls Profile, Inc., a Burbank, Calif., firm that manufactures small machine-tool parts for aircraft and missiles. He also is senior vice president of Southern Pacific Title Co. in Santa Ana, Calif.

Earlier in the day, the White House had defended Michael Reagan's action and said it would provide informal advice to all four Reagan children to help them avoid "even the appearance of impropriety" in the future.

"I don't think the president has any problems with the way Michael is doing business," White House deputy press secretary Larry Speakes said when first questioned about the letters.

But later, after White House aides apparently had given further consideration to the possible ramifications of the matter, Speakes issued a statement saying that White House counsel Fred F. Fielding would "provide informal guidance to Michael and [the] other children from time to time."

The statement said Michael had asked for such advice, "In president's view," it added, "this precautionary step is the best way to ensure that his children can continue to exercise their full rights as private citizens while also observing their special responsibilities as members of the first family, thus avoiding even the appearance of any impropriety."

In his early remarks, Speakes said White House officials hope that Michael Reagan "can do business without that [his relationship to the president] coming up all the time." But repeatedly referring to the younger Reagan as "a private citizen" with a right to conduct business, Speakes indicated there is no White House opposition to a continuation of business dealings with government agencies by the president's son.

Asked whether guidelines issued last year that prohibited must business dealings with the government by relatives of the president applied in the Reagan administration, Speakes replied, "Those were Carter guidelines." tThe guidelines were issued by then-president Carter in response to the controversy over his brother Billy's dealings with the government of Libya.

In March, Michael Reagan, 35, sent letters to procurement officers at eight to 10 military installations around the country expressing interest in doing business with the bases and noting that "with my father's leadership at the White House, this countries [sic] Armed Services are going to rebuilt and strengthened."

Dana Ingalls, the head of the company, said yesterday that young Reagan had been on the firm's payroll since Jan. 1. "All we're trying to do is make a living," he said. "But I guess we won't be writing any more letters."

Speakes said that Michael Reagan called the president on Wednesday and told him about the letters because by then the younger Reagan "was being hounded by members of the media." He said the president "feels that Michael is a private citizen and should have the opportunity to do business without the problem of the family connection interfering."

The spokesman also noted that the business being sought by the Ingalls firm is awarded on the basis of competitive bidding and that in his opinion Michael was not trying to trade on the president's name."

In another development yesterday, a spokesman for the Los Angeles County district attorney's office said the prosecutors are "very close to making a decision" on whether to bring charges against young Reagan for allegedly diverting $17,500 from investors in a gasohol venture to pay for his travels during his father's 1980 presidential campaign. The prosecutors have been investigating the alleged violations of California securities laws.