An unwritten White House policy that President Carter will not personally receive visiting opposition political leaders has caused West German Christian Democratic Party leader Helmut Kohl to cancel his visit to Washington.

The policy, never announced, has caused considerable disappointment at the top of the West German party - which actually commands more votes than the ruling Social Democrats - and concern among the European politicians as well.

Sources close to Kohl confirmed that he was unable to get an appointment to see President Carter. Kohl reportedly has been trying since the summer. He has now canceled plans for the U.S. visit feeling that it would be an embarassment at home for a politician of his stature to come to Washington without seeing Carter.

West Germany is probably the most important economic and military ally of the United States in Western Europe and and its opposition political leaders have traditionally had access to American presidents. Kohl visited with President Ford during a visit last year.

Authoritative German sources say Kohl has been told officially that there was no time available on the President's calender thus far. Informally, aides say, he has been told by Washington officials that the real reason is the White House decision to funnel opposition leaders to Vice President Mondale.

[At White House spokesman confirmed that Kohl was offered an appointment with Mondale, but the official said he was not prepared at this time to affirm existence of a presidential policy not to receive opposition leaders.]

German politicians call such a policy "foolish," a view shared by some other European diplomats here who are aware of the situation.

They believe such a policy is short-sighted since it could create hard feeling among opposition political leaders, who may be in power soon. They also feel it tends to isolate the President from the ebb and flow of differing trends in these countries.

It also can isolate foreign opoosition politicians from other U.S. policymakers, who may take their cue from the White House, or simply cause more cancellations of trips, as in Kohl's case.

One aim of the White House policy is presumably not to increase the stature of a shadow government in relation to incumbent governments with which Washington is dealing. Another, perhaps, is to reduce the already heavy demands on the President's time.

The source of the White House policy reportedly was French Socialist leader Francois Mitterrand's desire to see Carter earlier this year. At the time, French President Valery Giscard d'Estaing appeared to be in danger of being ousted in elections next year by a Socialist-Communist coalition and the White House did not want to colster Mitterrand's prestige. It did not allow the meeting. The basic policy was established after that decision, the sources say.

In September, Carter did meet with British opposition leader Margaret Thatcher, to the chagrin of White House officials. That visit, it was explained, resulted mostly from a perhaps unguarded response from the President during his trip to Britain in May when Thatcher remarked that she was looking forward to seeing him in Washington.

In West German elections last year, Kohl's Christian Democrats and their sister party in Bavaria, the Christian Social Union, polled 48.6 per cent of the vote, considerably more than the 38 per cent recorded by the Social Democrats of Chancellor Helmut Schmidt. The Social Democrat's alliance with the small Free Democratic Party gave that left-center coalition enough votes to stay in power.

There is some irony in Kohl's inability to gain entry to the Oval Office. During campaigning here at the same time as the U.S. presidential race. Kohl's chief strategists were intrigued by candidate Carter and attempted to adopt his strategy.

Thus Kohl was pictured as trying to "run against Bonn" as Carter was running against the establishment in Washington Like Carter, Kohl was portrayed as one who was better equipped to find solutions and who was closer to basic values and political attitudes of the voters.