The space telescope that space agency chief James M. Beggs believes will be "the most important scientific instrument ever flown" has experienced so much developmental difficulty that it will cost $200 million more and reach earth orbit a year later than expected, according to a House staff report.

The report concluded that the space telescope will cost the National Aeronautics and Space Administration $1 billion by the time it flies. Its current estimated cost is $797.2 million, which is $225 million more than expected when the program began in 1977.

The report also concluded that the space telescope will be carried into earth orbit by the space shuttle no earlier than April, 1986, a year later than planned. If it reaches earth orbit any later, the telescope will miss Halley's Comet when it swings around the sun that year. Halley's Comet was to be one of the telescope's first demanding assignments.

The report, by the House Appropriations Committee's surveys and investigations staff, said that there are so many "outstanding questions" about the design of three "fine guidance sensors" for locking the telescope onto distant stars that an internal review committee has recommended that the half-completed instrument be tested for at least a year and that a new instrument be built based on the test.

The device is being developed by Perkin-Elmer Corp. of Danbury, Conn. Perkin-Elmer and California's Lockheed Corp. are the project's two major contractors.

"If this recommendation is accepted, a one-year delay in the program will occur, costing $100 million," the report said. "A launch will then be delayed until at least April, 1986."

A device called the orbital replacement unit, for aligning and securing the scientific instruments in the telescope, must also be redesigned, the report said.

"If a quick-fix proposal, tentatively costed at $6 million, is not successful, a redesign may be necessary and a potential transfer of Orbital Replacement Unit latches responsibility from Perkin-Elmer to Lockheed is being considered," the report said.

The third "outstanding question" that must be answered, the report went on, is whether to clean the telescope's primary mirror. The report said the mirror has accumulated so much dust after 15 months in a Perkin-Elmer "clean room" that it already fails to meet the mirror's original cleanliness specifications and has lost 20 to 30 percent of its reflecting power.

"The questions are, what is the expected amount of additional dust accumulation over the next several years of storage before launch," the report said, "and if NASA decides to clean the mirror, what is the best cleaning method to use?"

The report put most of the blame for the delays and cost overruns on NASA for understaffing the program by 50 percent in its early development years and on Perkin-Elmer Corp., which it said "failed to recognize and properly plan for a project of the technical and manufacturing difficulty of the Space Telescope." As a result, the report said, the company "engaged in long-lived 'crisis management' of the project."