The U.S. ambassador here, John J. Louis Jr., announced today that he is leaving the post and made it clear he is doing so against his will.

"My wife, my family and I are disappointed and saddened at the prospect of leaving Britain," Louis said in a statement issued by the embassy after a London newspaper report said Louis was to be replaced because he had been "lacking in impact" as the envoy here.

So abrupt is the change that Britain has not had time to approve Louis' designated successor, sources said. He is understood to be Charles Price, a banker and candy company executive from Kansas City, who is ambassador to Belgium.

Louis was named to London by President Reagan at the outset of the administration. He is an heir to the Johnson's Wax fortune and was a major contributor to Republican politics. But he had no diplomatic background and little public experience.

In his tenure here, he has proven a willing and gracious host and from all accounts has fulfilled his public relations functions adequately.

But his role on substantive political matters has been limited, and he raised official eyebrows on both sides of the Atlantic by failing to return to London from a vacation until 10 days after the start of the Falkland Islands war in the spring of 1982.

Louis' statement said he is leaving with "a great sense of accomplishment. Anglo-American relations are excellent . . . . Britain's support for alliance policies and the effectiveness of our consultations have allowed the United States and the United Kingdom to work together with renewed strength . . . ."

Despite the unmistakable tone of regret at being replaced, Louis concluded his statement saying: "Our responsibilities here have been the most demanding, the most thrilling and the most satisfying of our lives."

There was no indication in the statement or from U.S. and British sources why Louis' resignation was requested. The gossip column that reported Louis' departure, written in the Daily Mail, suggested that Prime Minister Margaret Thatcher's "enthusiasm for even closer ties with the United States clearly needs a professional ambassador."

If that is the case, Price will not represent a major change, at least in terms of his personal background. Like Louis, he is a substantial donor to GOP campaigns and with his wife, Carol, has a reputation in Kansas City as a party-giver of considerable note.

As ambassador to Belgium, however, he apparently has impressed those he worked with by his interest in issues of foreign and domestic policy.