Conventional circles may think it's more or less a new trend, but to Sweden's Prince Bertil it's an old story -- a 34-year-old old story. That's how long he lived with the woman he loved before they were allowed to say their "I do's" in 1976.

"That's a world record, 34 years living together and not being married," joked the tall, handsome 70-year-old prince, who had been second in line of succession to the Swedish throne until his nephew, Carl Gustaf, had an heir. "That's disgusting!" he said of his record with a smile.

Bertil let reporters in on what had been, for a third of a century, one of Sweden's worst-kept secrets, at a National Press Club luncheon yesterday as part of the "Scandinavia Today" launching. He explained that his father, Sweden's late King Gustaf VI Adolf, wouldn't let him marry until his nephew, now King Carl XVI Gustaf, had come of age and was married.

"When King Carl Gustaf got married I said to the young king, 'Listen, now that you can have an heir I'm going to get married whether you like it or not,' " Bertil told a sellout crowd taking in the second day of the Scandinavian royal roadshow, which has been wowing Washington as it opens the 15-month-long celebration of Nordic cultural events that will tour six American cities.

All of this titillated the journalists, who burst into uproarious laughter and applause at Bertil's candid response to a question sent him from the floor. If he were growing up today, began the question read to him by press club president Vivian Vahlberg, did he think he would have to wait 34 years to marry his sweetheart?

"Now what do you know about that?" he deadpanned before astonishing his American audience with the unexpected explanation.

Bertil's wife, Princess Lilian, the English-born divorced commoner whom Bertil first met in wartime London when she was a nurse and he was Sweden's naval attache, smiled unself-consciously from underneath her flowered pillbox hat.

Looking on from the head table were Norway's Crown Prince Harald and Crown Princess Sonja; Iceland's President Vigdis Finnbogadottir; Finland's Foreign Minister Pers Stenback; Denmark's Cultural Minister Lise Ostergaard; and the ambassadors of Sweden, Norway and Denmark.

If yesterday seemed to be Bertil's turn to captivate a Washington audience, much as President Vigdis had done the previous day, there were still plenty of laughs to go around for everybody in the Scandinavian party during the post-luncheon question-and-answer period.

Said President Vigdis of President Reagan, who entertained her at the White House Wednesday: "A very charming person, of course, coming from the theater." And on being a woman president: "I realized it was a disadvantage when everybody expected me to be like a man. After the election, it was a great advantage because it put the country in the limelight."

Said Crown Prince Harald of Scandinavian monarchies -- once described as the supreme example of bicycle-and-umbrella monarchies sans glitter: "First of all, I don't have a bicycle."

Said politically neutral Finland's Stenback, when asked who serves the best meals -- the Kremlin, the White House or the National Press Club: "I like lobsters, I like caviar but in the presence of my dear wife, I must say that her cooking is best."