Britain today joined a growing number of International Whaling Commission members that support a worldwide moratorium on commercial whaling.

For the first time in five years, several moratorium proposals, including one from the United States, are on the agenda of the Whaling Commission, which has begun its week-long annual meeting here. Environmentalists of the "Save the Whales" campaign and members of the U.S. and other delegations supporting a moratorium are optimistic that at least a limited ban of some kind may be approved this week.

The U.S. proposal is the strongest. It would ban all commercial whale hunting "until there is an effective, comprehensive conservation program for whales which will guarantee their continued survival."

Australia proposes making a study of whether whaling should be banned forever. And the small Indian Ocean island nation of The Seychelles, which joined the Whaling Commission to press conservation measures, proposes that sperm whale hunting be stopped for at least three years and that the entire Indian Ocean be made a whale refuge.

Although a majority of Whaling Commission members already favors a moratorium, approval by three - fourths of the 23 nations is required for adoption.

Several nations still heavily engaged in commercial whaling, including the Soviet Union, Japan, South Korea and Peru are certain to oppose any form of moratorium. The chances of working out a compromise rest with undecided nations like Norway, Denmark, Iceland, Spain and Brazil, which have been under intense pressure from domestic and international environmentalists.

In an opening address today, Alick Buchanan - Smith, an agriculture minister in Prime Minister Margaret Thatcher's Conservative government, announced that Britain would both support a commission moratorium on commercial whaling and ask the Common Market to ban all imports of whale products. The United States already bans such imports and outlaws whale hunting in U.S. waters and by U.S. companies or citizens.

The United States, Britain and other nations seeking a moratorium believe that the Whaling Commission's annual kill quotas have failed to prevent the serious depletion and near extinction of a number of whale species. Those nations complain that the commission does not receive sufficient data from whale - hunting nations or the commission's own experts. Moratorium proponents also charge that commission quotas are widely violated by non - member "pirate" whales they kill "illegally" to member nations, especially Japan.

Japan, which had long denied charges that it was circumventing the commission's catch quotas, announced last week that it would ban all imports of whale products from nations that are not commission members.

Representatives of many international environmentalist groups seeking protection of whales are monitoring the meeting and lobbying undecided delegations. Yesterday, about 7,000 people attended a "Save the Whales" rally in Trafalgar Square. Today, several hundred protestors chanted and waved banners from behind police lines across Regent Street from the commission's meeting place in the commercial center of London.

At this morning's opening session, singer John Denver gave the commission a petition signed by half a million Americans urging a ban on whaling, told the delegates about his experiences swimming with whales, and sang a song, "I Want to Live." CAPTION: Picture, Demonstrators display signs and models of whales outside the International Whaling Commission meeting.