The enigma of one of the rarest photographs in British theatrical history was solved yesterday by an eyewitness account of the mood in which it was taken nearly 21 years ago.

The picture, to be auctioned for charity at Christie's in London tonight, is the only known image of Sir Ralph Richardson, Lord Olivier and Sir Alec Guinness in one frame.

The answer to the enigma, discussed by the Guardian when the photo's existence was revealed last week, is that the spirit behind their pose is "irrepressible" mischief rather than the infirmities of their old age.

This was partly because they felt naughty being photographed together without Sir John Gielgud, the fourth great theatrical knight of the late 20th century.

"Johnny would be annoyed if he knew about this," said Sir Ralph. So they asked the photographer, Terence Donovan, a close friend of Sir Ralph, to promise that the picture would be kept secret until they were all dead. It came to light only when Donovan's widow, Diana, gave it to the HIV charity London Lighthouse to raise funds for tonight's auction.

The photo session was in May 1980 at Holborn Studios, London, officially to take individual shots of the three for Vogue magazine. All had been film idols in their youth, but Richardson was then 79, Olivier 73 and Guinness 66.

Vince McCartney, now studio director, was at the session. "We had two girls of about 18 and 25 in reception, almost too young to have heard of the three of them. But when they walked in, doffed their caps with their canes in their hands and said good morning, they had such star presence that the girls almost passed out," he said.

"It was overwhelming. They were as frisky and nimble as boys playing truant from school. There was lots of fun."

Lord Olivier, who had been seriously ill with cancer and other conditions for several years, told Mr McCartney: "You know, my boy, I'm dying."

Sir Alec put in: "Yes, but you must remember that he has been saying that for at least 10 years."