A member of the British royal family, in a major break with tradition, will leave the Church of England and join the Roman Catholic Church, becoming the first member of the royal family to convert to Catholicism since King James II in the 17th century.

Katherine, the Duchess of Kent, whose husband is 18th in line to the throne and a cousin of Queen Elizabeth II, was to convert to Catholicism Friday evening in a private ceremony conducted by Car-dinal Basil Hume at his chapel in the archbishop's house in Westminster.The decision is a major break with tradition because the British monarch is the titular head of the Church of England and would be disqualified from the throne if converting or marrying someone of a different religion. The break prompted some observers to speculate that a separation of church and state might occur within a few years.

The duchess's decision, made public earlier this week, had been expected for some time because she has maintained a close relationship with the Catholic Church for several years.

"The Duchess of Kent has for some time taken instruction with a view to joining the Roman Catholic Church," a spokesman for the duchess said in announcing her decision earlier this week.

"Later this week, in the private chapel of the archbishop's house of Westminster, her highness will be received as a Roman Catholic. This is a private matter and has no institutional implications whatsoever," he said.

Some royalty watchers, however, believe otherwise. Harold Brooks-Baker, publishing director of Burke's Peerage, said he saw the conversion as a clear move toward the separation of the Church of England and the state.

"This kind of announcement would never had been made five years ago," Brooks-Baker said.