John Jeffry Louis, 54, an Illinois communications executive and scion of the Johnson's Wax fortune, has been selected to be President Reagan's ambassador to Great Britain, according to informed sources.

White House press aide Karna Small said she could neither confirm nor deny the appointment until an official announcement had been made. This is among the first ambassadorial appointments of the new administration and, after rhetorical homage to a merit system concept by the Carter administration, it reinforces the tradition in which presidents award political backers with diplomatic posts.

Louis, chairman of the Phoenix-based Combined Communications Inc., a wholly owned subsidiary of the Gannett communications empire, has been a heavy contributor to Republican candidates, though he was not "inordinately active" in Reagan's 1980 campaign, according to an aide to the president.

Louis' background in international relations consists largely of a stint in international marketing for his mother's family's wax business, based in Racine, Wis., in the late 1950s, according to his longtime friend and associate Karl Eller of Phoenix, former head of Combined Communications.

Eller, now owner of Swenson's Ice Cream and president of Columbia Pictures Communications, acknowledged that he is a substantial financial backer of the Reagan campaign.

Eller said he was consulted about the ambassadorship for Louis but did not play a direct role in securing the post for his friend.

The record of Louis' contributions to the Reagan 1980 campaign was not immediately available. But according to campaign records of 1972, Louis was the fifth-largest contributor to President Nixon's reelection campaign, when he was listed for $286,726 (including $20,000 given by his mother, Henrietta Johnson, and two brothers).

Eller and Louis were partners in an advertising business in the early 1960s, and in 1968 founded Combined Communications. The company owned the Oakland Tribune and the Cincinnati Enquirer. In 1979, in the largest media merger in history, it was taken over by Gannett. Louis serves on the Gannett board of directors.

Louis has served as an investor of his family's money, Eller said, and also has considerable background in "helping to run schools." He has been a trustee for Northwestern University since 1967 and has also served Deerfield Academy, Foxcroft School and Williams College in a similar capacity.

Louis also was one of three owners and directors of the Atlanta Braves baseball team who, in 1969, were orderd by the baseball commissioner to get rid of stock ties to Las Vegas gambling casinos. Louis was then on the board of the Parvin-Dohrmann Co., owner of three Las Vegas hotels and casinos.

Louis, a resident of Winnetka, Ill., could not be reached for comment.