BONN — West German Finance Minister Gerhard Stoltenberg said Monday that his government has decided that a controversial chemical plant in Libya was built with West German help and was designed to make poison gas.

Stoltenberg's comment was the first indication that a senior member of the West German government has come to accept that the plant was designed to produce chemical weapons, as the United States has charged. And it follows a series of disclosures that a number of West German companies may have played important roles in the construction of the plant.

At the same time, a prosecutor in Belgium said that the owner of a Belgian shipping company has admitted sending to Libya the construction material, equipment and chemicals for a complete chemical plant.

Stoltenberg told a news conference in Bonn that "we have to assume that at this factory there is a section that will be able to produce poison gas." He said the assumption "is based on concrete indications and reports."

Denials From Bonn at First

Libya insists that the plant in question--at Rabta, about 40 miles southwest of Tripoli--was designed to produce only pharmaceuticals. For a time, officials of the West German government, including Chancellor Helmut Kohl, seemed to accept this, and Bonn denied there was any evidence that West German companies had provided plans or supplies for the chemical complex.

But Washington has pressed Bonn with evidence in support of its charges that West German firms have helped set up the plant and that it was designed to be a major producer of poison gas. In recent weeks, statements from West German officials grew contradictory. And last Friday, Friedhelm Ost, chief spokesman for the Bonn government, said officials had received "serious information" that a West German firm, Imhausen-Chemie, was involved.

Imhausen-Chemie had emerged from a Finance Ministry investigation with what appeared to be a clean bill of health, but Ost's comments made it clear that the firm was under new suspicion. He said evidence implicating Imhausen-Chemie had been disclosed as long ago as last October.

Stoltenberg indicated Monday that the BND, the West German intelligence agency, had reported to Bonn on the Libyan plant's weapons-making capability, but he said the BND had "emphatically urged" the government not to undertake a public investigation of Imhausen-Chemie because intelligence sources would be compromised.

Vague Reports