The Supreme Court ruled yesterday that police interfered with a suspect's right to a lawyer when they planted an informant in his jail cell.

In the 6-to-3 ruling, the court said paid informants may not be used to elicit information police themselves cannot lawfully obtain.

The court overturned a Virginia man's federal bank robbery conviction gained by use of incriminating statements made to a government informant sharing the suspect's cell.

"By intentionally creating a situation likely to induce [the defendant] to make incriminating statements without the assistance of counsel," Chief Justice Warren Burger wrote for the court, "the government violated [his] Sixth Amendment right to counsel."

Justice William Brennan, Potter Stewart, Thurgood Marshall, Lewis Powell and John Paul Stevens joined Burger in the case of U.S. vs. Henry Justice Harry Blackmun, Byron White and William Rehnquist dissented.

In other action yesterday:

The court upheld exisiting water rights in most of California's fertile Imperial Valley.

In a unamimous decision, the justices reversed a 9th U.S. Circuit Court of Appeals ruling limiting the amount of land that could be irrigated with Colorado River water to parcels of no more than 160 acres.

The high court noted that the lower court ruling would probably have forced the sale of large amounts of land at below-market prices.

About 223,000 acres today are held in farms larger than 160 acres in the area. Many are owned by big corporations.

A number of Imperial Valley residents who wanted to buy this land had brought suit charging a violation of the Reclamation Act of 1902, which authorized vast amounts of government spending for dams and canals to irrigate dry western states.

The act restricted the water to plots under 160 acres. But Justice White, writing for the court, held that the Imperial Valley land was exempted from the restriction under a 1929 act of Congress.

Congress is now working more generally on the 160-acre limitation in a bill that passed the Senate and is in committee in the House.

The court rejected an effort by the Machinists Union to avoid complying with a Federal Election Commission subpoena issued in connection with union spending on the presidential campaign of Edward M. Kennedy.

The FEC began its investigation into the Machinists Non-Partisan Political League after campaign officials for President Carter complained that it had violated federal campaign laws by exceeding the $5,000 ceiling on contributions to a single candidate.

The commission issued a subpoena for records, which the Machinists fund resisted. A lower court upheld the subpoena and yesterday the Supreme Court, without comment, rejected the union's appeal.

The court also rejected a challenge to the 1980 census by 22 members of Congress and two citizen organizations. The groups -- led by the Federation for American Immigration Reform -- had sought separate tallies for illegal aliens.

A lower court ruled in February that the protesting organizations had no right to sue since they could not show any "concrete harm" to them from the census.

The justices declined yesterday to review that ruling.

The court agreed to hear arguments in a California case challenging open space laws. The San Diego Gas and Electric Co. sued the city of San Diego, charging that the city's open space measure deprived the firm of land it was going to use to build power plants.

The company demanded compensation but was rebuffed by the California Supreme Court.

The justices ruled just last week against landowners making similar claims against the California city of Tiburon. The landowners in that case -- Bonnie and Donald Agins -- had never actually applied to build on the affected property, however, and the court said it could not rule on the larger issues.