The world's leading industrial nations agreed at their recent meeting in London that a further moderate decline in the value of the U.S. dollar would benefit the global economy, according to a senior Reagan administration official.

The official, in an interview earlier this week, said reports that there was no longer unanimous support among five leading industrial nations -- known as the Group of Five -- for a further reduction in the value of the dollar were "erroneous."

Finance ministers and central bankers from the five nations -- the United States, Japan, West Germany, France and England -- met in London on Jan. 19.

In an interview Monday, the official said "there was a feeling on the part of everybody that there would be some more room for the dollar to decline , provided it was gradual, and moderate, and in response to market forces."

The senior official refused to be drawn into a discussion of current intervention policy designed to depress the dollar's value. He only said that the United States and the other nations would continue "to consult and coordinate with each other with respect to exchange rates. Whether we intervene or not is a matter we ought not to get into in public."

However, the official's stress on the "response to market forces" seemed to imply that the coordinated intervention to push the dollar down, which had been agreed upon in New York at the Sept. 22, G-5 meeting, was not extended in London. Japanese Finance Minister Noboru Takeshita similarly emphasized "market forces" in an interview last week.

According to the administration official, the G-5 group meeting in London also:

*Discussed the need for greater economic expansion by Japan and West Germany. "We'd like to see Germany and Japan expand more," he said. "Germany has done some things on its interest rate targets, a modest amount on monetary targets. But they're not particularly willing to speed up their tax cuts."

*Reviewed the Third World debt problem. The ministers agreed it had become counterproductive to talk about which debtor nations might be first to qualify for additional loans under the so-called Baker debt initiative. In a speech last October, Treasury Secretary James A. Baker III had urged an expansion of both commercial and multilateral development bank loans to debtor countries that reformed their domestic economies.

Prior to and just after the London meeting, it had been reported that some of the United States' trading partners, including Japan and West Germany, were not anxious to see their currencies appreciate further against the dollar.

But Japanese officials appear to have shifted their policy over the past several days. Some reports from Tokyo suggest that Japan may be willing to accept the costs of a higher yen -- which should reduce export surpluses -- because officials hope that lower oil prices will reduce the cost of imports, and at the same time help spur their domestic economy.

Since the London session, the dollar has continue to decline, especially against the yen, in response to signals from Japanese officials that they were willing to see their currency appreciate from around 200 to $1 at that time to the 190 to $1 level. The yen closed yesterday at 195.40 to $1 in Tokyo.

The administration official implied that the consultative process deepened by the G-5 in New York was pretty much intact, although the finance ministers and central bankers -- sensitized by other nations' complaints that they are being excluded -- now seek a lower profile for their group.

Private G-5 sessions will continue. But there will be renewed emphasis on the Group of Ten. This group actually has 11 members: the Group of Five, plus Canada, Sweden, Switzerland, Italy, Belgium and Holland.

U.S. Treasury officials would not comment on a recent report in The Washington Post that the president had asked Baker to look into ways of promoting greater stability in international currency relationships.

The Interim Committee had already scheduled a further discussion of two reports relating to currency reform: one developed in April 1985 in Tokyo by the G-10 deputies, which suggested that very few practical changes could be made in the system; and the other by a group of Third World countries, known as the G-24, which argued for major changes to curb volatility.

Baker's Treasury is known to be looking for some practical ways of reforming the system, because it is convinced that excessive volatility of currencies had exacerbated America's trade deficit problem.

At the moment, the Baker Treasury is interested in preventing a reversal of the gains registered since the New York G-5 meeting: it not only would like to see a further gradual decline of the dollar, but would like to ensure against a reversal of the process that would drive the dollar up sharply.