The actor Wendy Craig once starred in a sitcom with a theme song that declared love was like a butterfly, as soft and gentle as a sigh. In Craig's case it appears that the butterfly was very literate and somewhat randy, according to weekend reports.

Craig had an affair with the playwright John Mortimer more than 40 years ago, which produced a son, he confirmed yesterday.

The lovers were married to others at the time, and Mortimer, author of the Rumpole of the Bailey books and now knighted, only found out a year ago that the affair had produced a child, Ross.

He was told by Craig, who feared that the child might become public knowledge.She and her husband, Jack Bentley, brought up Ross as their own. Bentley died in 1994.

Craig, now 70, went on to star in several television series, including Butterflies, where she played a housewife toying with having an affair.

Sir John, 81, is one of the country's most successful playwrights and authors.

He said the affair began when Craig acted in his play The Wrong Side of The Park: "It was the 60s and we were all a lot more excitable then." Craig has declined to talk about the affair.

Sir John told the Sunday Telegraph he met Ross for the first time earlier this year "and got along very well with him. He met the rest of my family over lunch and got along well with them, too. We all hope to see a lot more of him."