President Reagan has decided to build the MX missile and deploy it in Nevada and to start production of the controversial B1 bomber, according to administration officials.

Asked whether Reagan had opted for the widely expected deployment of 100 MX missiles shuttling among as many as 1,000 concrete shelters and a go-ahead for production of as many as 100 B1 bombers, senior administration officials said that was "not totally correct." They refused to pinpoint what was correct and what was inaccurate.

Sen. Paul Laxalt (R-Nev.), after checking with the White House, also said such reports contained inaccuracies.

However, other officials said the rotation of about 100 MX missiles among as many as 1,000 shelters is the core of the president's plan.

The State Department has drafted messages to allied governments mentioning the 100 missiles and 1,000 shelters, sources said. As of late yesterday, these messages remained unsigned and unsent, however, leaving room for last-minute alterations.

One reason Reagan is holding up formal announcement of his decision until Friday is to inform allied governments privately of his plans.

The decision is Reagan's major effort to close what he calls "the window of vulnerability" to a possible attack by the Soviet Union. The perceived vulnerability to a Soviet first strike results from improvement of Soviet warheads to make them accurate enough to blow up U.S. missiles in underground silos.

Officials said Reagan has chosen a scaled-down version of the MX plan approved by the Carter administration, rejecting Defense Secretary Caspar W. Weinberger's preference for deferral of a decision on deployment of the MX missile for several years.

That would allow further research and development work on an antiballistic missile (ABM) and a smaller so-called "common missile" that could be used jointly by the Air Force, which controls land-based missiles, and the Navy, which has missiles aboard nuclear-powered submarines.

In addition to the MX and B1, Reagan reportedly has decided to go ahead with accelerated research and development on antiballistic missiles, on the D5 Trident II missile for the Navy, on building very deep underground protective shelters for possible future land-based missiles and on a method of basing missiles in planes that could keep them aloft for long periods, officials said.

The president also has approved a major program to strengthen the presidential and military command-and-control system that would be the communications network during a war and to upgrade the nation's air defense system.

Reagan is scheduled to announce his package of interrelated decisions Friday, ending months of intense study within the administration.

White House communications director David Gergen underlined the importance the administration places on these decisions by telling reporters he could think of no precedent for a president at a single stroke making so many decisions with such long-range importance for the nation's national security.

The MX and the B1 have been by far the most controversial parts of the package. Both were the subject of intense debate during the Carter administration.

Former president Carter decided not to build the B1, saying it would be obsolete as soon as it was produced and chose to move directly toward the more advanced Stealth bomber.

In reversing that decision, Reagan has sided with the Air Force and several congressional defense experts who have pressed for construction of the B1.

Reagan's decision, according to administration officials, is to move ahead with both planes. One hundred B1 bombers would be authorized at a likely cost of about $200 million each. After 50 are built, the second 50 could be canceled if Stealth is ready, officials said.

The major argument against the B1 is that once it begins to fly in 1986, Soviet air defenses may have improved to the point it is an easy target.

Some critics also wonder whether the United States can afford to pay for the B1 and Stealth at the same time. Stealth is reported to be costing more than $1 billion a year, with that amount expected to increase.

Reagan apparently rejected these arguments and sided with those who have argued that the aged B52 bombers cannot be kept flying safely as the nation's strategic air arm until Stealth is ready. In a conventional war, B1 advocates argue, a new bomber would be needed.

Reagan, like Carter, wrestled for a long time with the MX decision. Each administration has reached a decision that it admits is less than perfect.

By accelerating research and development for deep underground silos, ABM and air-basing of strategic missiles, Reagan signaled that he intends to continue looking for a better way to deploy missiles than the shell-game shuttle in Nevada.

Pentagon research chief Richard DeLauer told reporters Aug. 25 that Weinberger wanted to delay an MX basing decision until more work had been done on a new generation of ABMs. That postponement would have saved Reagan money in the near future that would have helped his effort to cut the federal budget.

Reagan's decisions to push ahead with new missiles precede the new administration's move to resume strategic arms limitation talks with the Soviet Union. Any new missiles would be new subjects for negotiations if talks aimed at reducing the superpowers' nuclear arsenals are restarted.

Reagan was under pressure from European allies not to defer MX deployment because he is seeking to base new nuclear weapons there and European protesters would seize on any sign that he would halt missile deployment here while pushing ahead in Europe.