Britain, France and Italy have agreed to have their navies work together to sweep mines out of the Persian Gulf, defense officials disclosed yesterday.

The pact calls for the three navies to take turns sweeping mines in such a coordinated pattern that at least five mine sweepers will be on duty at all times in the gulf, officials said.

Reagan administration officials said they are encouraged by this multinational approach to keeping the strategic waters of the gulf open to the world's shipping. They said the agreement represents an unprecedented degree of allied cooperation in the gulf region. They added that the administration hopes this will blunt congressional criticism that the United States is bearing an inordinate burden by spending at least $1 million a day to protect the gulf.

Also, sources said, the more active role by these allies -- Britain and Italy are full NATO partners while France left the military command more than 20 years ago but has remained a member of the alliance -- comes at a time when the Reagan administration is looking for graceful ways to reduce U.S. military presence in the Persian Gulf region.

Soon there will be a highly visible decrease in U.S. firepower there when the battleship USS Iowa heads for home, Navy sources said. The giant ship has been on patrol in the northern Arabian Sea below the gulf. The Iowa is armed with Tomahawk cruise missiles programmed to hit military targets in Iran, officials said.

Other U.S. ships to remain in the gulf also have Tomahawks with software tailored to Iranian targets, they added.

Defense Secretary Frank C. Carlucci and Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff, are expected to describe the forthcoming departure of the Iowa as a long-scheduled rotation, not a reduction of U.S. forces in the gulf. Military planners, said this is indeed the case but the long-range plan is to reduce gradually the Navy's presence in the region.

Last July, when the U.S. Navy started escorting Kuwaiti tankers flying the American flag, the Joint Chiefs of Staff figured eight warships plus the command ship LaSalle would be enough to combat the Iranian naval threat. Since then, the Navy has had as many as 30 warships on duty in the gulf region. The departure of the Iowa and the rotation of smaller ships out of the gulf over the next several weeks will result in only a slight reduction, officials said.

The British, French and Italian mine sweepers will shore up what has turned out to be one of the U.S. Navy's biggest shortcomings in its gulf operations. The Kuwaiti tanker Bridgeton, sailing under the U.S. flag, struck a mine in the gulf on July 24 during the Navy's first escort operation. There was an embarrassing scramble after the attack to find mine sweepers to combat this element of the Iranian military threat.

Italy's decision to participate in the mine-sweeping effort is likely to draw political criticism at home, officials said. The Italian Communist Party has been protesting the deployment of any part of the Italian navy to the gulf.

In the past week, British, French and Italian naval commanders in the area have declared that gulf waters are for the moment virtually clear of mines after four months of intensive mine-sweeping operations.

The decision of the three nations to cooperate was taken by western officials as a signal that the European navies were preparing to withdraw some of their forces and consolidate others in a cooperative venture largely as a cost savings measure.

The last reported mine attack in the gulf was Sept. 22 when a small survey vessel, the Marissa I, struck a mine and sunk off the coast of Saudi Arabia south of Farsi Island.

Officials say there are no indications that Iran has engaged in the laying of mines since late September when U.S. naval and special forces detected and attacked an Iranian minelayer.

Military planners who have been studying the off again, on again tanker war in the gulf said the threat is now manageable by the allied forces deployed there. Not only is there a large number of warships, they noted, but the United States, with the help of several Arab countries, controls the skies. Warplanes on the deck of the USS Midway outside the gulf are on call as are Army helicopter gunships attached to Task Force 160, they said.

The Midway is scheduled to return home in a few months, Navy officials said, but probably will be replaced by another carrier. The Midway is operating in the Arabian Sea with a number of restrictions, they said, because it does not float high enough in the water despite an expensive modification of the hull to correct the problem. The carrier is not combat-ready all the time, sources said.

The Persian Gulf deployment could be the last for the Midway since it is possible that the Navy will retire the old carrier to save money.

A second carrier, the USS Coral Sea, also may be taken out of service soon to ease the Navy's budget crunch, officials said.

Staff writer Patrick E. Tyler contributed to this report.