The Secret Service is seeking punishment of a reporter who posed as a congressman to crack the supposedly tight security at the signing of the Israeli-Egyptian peace treaty on the White House lawn on March 26.

After investigating the incident, the Secret Service asked the Standing Committee of Correspondents of the Congress to take disciplinary action against the reporter, Gary C. Schuster, Washington bureau chief of The Detroit News.

It's not as if Schuster had attempted to hide his action. He wrote about it the next day, in a front page story in his paper under the headline "Crashing A Moment In History."

But what began as a lark is now being regarded as a serious, perhaps even criminal, action, by the Secret Service, congressional officials and even some of Schuster's journalistic colleagues.

Schuster told readers of the News that it was a "snap" to breach what was supposed to have been extremely tight security at the historic event.

"All it took was one call, one question, a little observation and one lie to get a police-escorted ride to a first-class seat on the executive mansion's lawn - 50 feet from the table where Mid-East peace documents were signed." Schuster wrote.

Accompanying Schuster's story was a picture of protesting Arab students trying to breach a police line outside of the White House.

Schuster's action was followed that same night by a report that four George Washington University law students posed as waiters and crashed the formal state dinner inside the White House honoring Anwar Sadat and Menachem Begin.

Benjamin C. West, superintendent of the House press gallery, said Schuster's gate crashing is being regarded as "a very grave matter with the potential for collective reprecussions" against all reporters accredited by Congress.

"It could very well involve violation of a federal statue," West said.

The five-member Standing Committee of Correspondents, which rules on requests for accreditation to the congressional press galleries and has the authority to strip reporters of that privilege for misconduct, met in closed session Friday to discuss the Schuster Affair.

Michael Posner, a reporter for Reuters, a British news agency, who is chairman of the committee, acknowledged that "serious charges" had been made against Schuster.

Posner said the committee agreed not to discuss the affair publicly until Schuster has been given a chance to explain his actions before the committee.

"This could ruin a guy's career." said Posner. "But the charges may be totally untrue."

Dick West of United Press Internationl, another member of the standing committee. said Schuster's privileges could be suspended for several months, an action that would bar him from observing floor action in either the Senate or the House from the press gallery.

"But the last ime something like that happened." the UPI reporter recalled, "the guy's paper paid him a bonus and he became a hero back home."

In his March 27 story, Schuster indicated that he got the idea for crashing the event after one House member told him he was not going to attend the signing, and that no special credentials had been issued for congressmen to board the buses for the trip from the Capitol to the White House.

"When more than 290 House and Senate members converged on the buses, police from the White House and Capitol were busy directing them to unfilled coaches - leaving no time for scheduled ID inspections," Schuster wrote.

"Instead, a security officer on board checked off each rider's name from a Congressional roster. By using the name of someone who wasn't aboard, the reporter was on his way. And because House members can't know each of their 435 colleagues, there weren't any questions about the stranger in their midst," Schuster wrote.

Yesterday Schuster declined comment on the Secret Service request and the Standing Committee deliberations. CAPTION: Picture, Gary Schuster: " . . . one call . . . a little observation, and one lie." Detroit News