A Reagan administration task force on violent crime has decided to recommend strengthening national gun control laws and giving the states $2 billion over the next four years to build more prisons.

The task force appointed by Attorney General William French Smith also agreed in meetings in New York last week on controversial proposals to deny bail to violent offenders and ease the rules against using tainted evidence.

The gun control recommendations may prove the most explosive politically because most are opposed by the National Rifle Association, a powerful lobby that considers Ronald Reagan an ally. During his campaign last year Reagan said he opposed gun control laws, and he has since expressed support for an NRA-backed bill that would weaken current laws.

The task force recommended a mandatory sentence for anyone using a firearm while committing a federal felony, which Reagan, the NRA and gun control groups all favor.

But the group also voted to propose amendments to strengthen the 1968 Gun Control Act. They would require individuals to report the theft or loss of a handgun to local authorities, make potential handgun buyers wait for records checks, and ban importation of unassembled handgun parts.

Pete Shields, chairman of Handgun Control Inc., said yesterday that he was delighted with the recommendations and hoped Smith and the president would follow through on them. All three are contained in a gun control bill sponsored by Rep. Peter Rodino (D-N.J.), chairman of the House Judiciary Committee, and Sen. Edward M. Kennedy (D-Mass.).

An NRA spokesman declined immediate comment, but noted the group has opposed similar proposals in the past. Carter administration Attorney General Griffin B. Bell, co-chair of the task force, said in a phone interview yesterday from Atlanta that the gun control proposals are "very modest. The NRA will have to give in some, too...They would be ill-advised to fight a modest thing like this."

Administration support for major federal prison spending also is uncertain. The proposal to give $2 billion to the states has not been discussed with the Office of Management and Budget, Jeffrey Harris, executive director of the eight-member task force, said yesterday.

White House counsel Edwin Meese III told the American Bar Association in New Orleans yesterday that it was "very unlikely" large amounts of federal money would be available to help states fight crime. One administration official said, "Frankly, this is one the administration will support in spirit, but perhaps not with money."

The $2 billion price tag on federal aid to state prisons was suggested by Illinois Gov. James Thompson, a Republican, the co-chair of the task force. States would have to pay 25 percent of the total construction cost and all of the operating expenses. Sen. Robert J. Dole (R-Kans.) has introduced a bill that provide $6 billion over six years for state prison construction.

Bell said, "If any administration is not willing to do what's necessary to attack violent crime, it won't be returned to office. This prison money is a key recommendation. It's peanuts compared to what the Defense Department spends."

The task force's final proposals, after four months of hearings around the country, will go to Smith next week. The recommendations the attorney general and the White House support will be made part of a legislative package this fall, Harris said.

"I can't say any one will make a difference in violent crime," he said. "We believe the entire package will."

Officials in both parties see a new attack on crime as an attractive political issue because the public ranks it next to inflation as a personal concern. The administration is considering a major push on the issue this fall, backed by a personal message from the president.

In June, a group of Senate Democrats endorsed a legislative package containing many of the same recommendations now being suggested by the task force.

David Landau, an attorney with the Washington office of the American Civil Liberties Union, said his organization opposed the no-bail recommendations and proposed changes in court rules to allow prosecutors to admit evidence that might have been obtained illegally. "They are singling out particular issues whose only impact will be on violating people's rights, not crime."