WASHINGTON (AP) _ The Reagan administration has decided to let Nicaraguan refugees remain in the United States indefinitely without fear of deportation, Rep. Joe Moakley said Friday, a move he criticized as ''purely political.''

Moakley, D-Mass., author of legislation to suspend deportations of Salvadoran and Nicaraguan refugees for two years, said he was told the administration would announce its decision within a week.

Despite Moakley's information, which he said came from a highly placed administration source, and numerous published reports this week, a White House official denied Friday that a decision had been made about the Nicaraguan refugee situation.

Immigration and Naturalization Service spokesman Verne Jervis said, ''There may be something to it but I don't have anything on it.''

A month ago, the Reagan administration turned down a request from El Salvador President Jose Napoleon Duarte that thousands of illegal immigrants not be forced to return to his country. The administration feared the move would hint that the Duarte regime, which it supports, is unstable.

Moakley said the decision on Nicaraguans who fled the leftist Sandinista regime there and illegally entered this country apparently will allow the refugees to remain indefinitely and be issued work permits.

He took issue with allowing Nicaraguans to remain, but not Salvadorans.

''Once again it shows to me that the administration is playing pure politics with the situation,'' Moakley said. ''I'm sure the only reason they're allowing the Nicaraguans to stay is they're coming from a communist nation and they're not letting the El Salvadorans because they say that's a democracy. It's a purely political situation.''

He added, ''They think they can treat one country one way, and another country another way.''

Moakley said the administration's decision apparently stops short of extended voluntary departure, a tool available to the White House to allow refugees to stay in the United States indefinitely. This status has been granted in the past to refugees from Afghanistan, Ethiopia and Poland who fled their homeland to avoid strife or persecution.

However, the administration has said repeatedly that granting extended voluntary departure status to the Nicaraguan refugees would draw other refugees like a magnet, some seeking jobs rather than political asylum.

The administration also has contended that allowing refugees from specific countries to stay in the United States would undermine immigration reforms approved by Congress last fall. That new law allows illegal aliens who arrived in the United States prior to Jan. 1, 1982, to remain.

However, many of the estimated 150,000 Nicaraguans in this country arrived after that date and, therefore, are not eligible for legalization.

The Moakley bill is expected to reach the House floor next month. ''I think they might be trying to submarine my bill and hoping if they give relief to the Nicaraguans then maybe some of the people supporting it for that reason won't support it anymore,'' Moakley said.