Prosecutors in the Iran-contra case of John M. Poindexter yesterday asked a federal judge to sentence him to prison, saying that the former national security adviser engaged in a "pattern of deceit" that "did violence to this nation's fundamental principles of government."

Poindexter was convicted April 7 by a U.S. District Court jury on all five felony charges lodged against him, including conspiracy and lying to and obstructing congressional inquiries into the Iran-contra affair.

The prosecution charged that Poindexter helped direct an illegal coverup designed to protect former president Ronald Reagan and to hide from Congress details of the administration's secret Iranian arms sales and support of the Nicaraguan contras during a congressional ban on U.S. military aid to the rebels.

In a five-page sentencing memo made public yesterday, independent counsel Lawrence E. Walsh and his four trial prosecutors said "appropriate weight should be given to the unambiguous nature of the jury's verdict and to {Poindexter's} position of high responsibility. . . . "

"If, as occurred in this case, high-ranking officials of one branch of government feel free to feed the other branch a diet of lies, then the constitutional system will surely sicken and, eventually, die," Walsh said.

U.S. District Judge Harold H. Greene is scheduled to sentence Poindexter on Monday.

Poindexter, 53, a retired Navy rear admiral, could receive a maximum sentence of 25 years in prison and be fined $1.25 million. He resigned as Reagan's national security adviser after the scandal erupted in November 1986.

The defense is expected to file its sentencing recommendation today. Poindexter plans to appeal his conviction.

None of the six others convicted for their role in the Iran-contra affair have received prison sentences, including former national security adviser Robert C. McFarlane and former National Security Council aide Oliver L. North.

Walsh noted the lack of prison sentences for North, McFarlane and the others but said that fact "does not in any way undermine the conclusion that the appropriate sentence in {Poindexter's} case must include some jail time."

North, who was convicted of three felonies, was fined $150,000, placed on two years' probation and ordered to perform 1,200 hours of community service work.

Walsh said U.S. District Judge Gerhard A. Gesell spared the former Marine a prison sentence partly because Gesell viewed North as following his superiors' orders.

Poindexter, Walsh said, was "one of those superiors."

Walsh said McFarlane's sentence was influenced by the fact that he pleaded guilty to four misdemeanors, cooperated with prosecutors and had attempted suicide.

Meanwhile, North testified for the second time yesterday before a new federal grand jury that Walsh is using to continue the Iran-contra investigation. Sources have said Walsh also is considering summoning Poindexter to testify before the grand jury.

The new grand jury's work includes examining whether other former Reagan administration officials not previously charged lied to Congress.