A spokesman for his agents said Fry had not been in touch since Saturday night and they did not know where he was. He was not responding to phone calls. "We are concerned about him," he said. Simon Gray, who wrote the play Cell Mates which co-stars Rik Mayall, said in a statement: "Our main concern is for his safety and well-being."
He added: "Three days after the successful opening of Cell Mates Stephen Fry expressed in a letter to me his great distress at what he considered to be his failure as an actor.
"It is obvious that he is a man in emotional turmoil, and that his sudden departure is the culmination of years of pressure, overwork, and unrelenting, self-sacrificing generosity."
Despite its frankness, the letter raised as many questions as it answered. Fry's agents, Hamilton Asper Management, said: "We're not in contact with him at the moment." A spokesman said he did not know of his client's whereabouts.
Michael Larkin, Mayall's understudy, said last night that the first the play's cast had heard of Fry's withdrawal was yesterday afternoon when they were told he was not coming back.
Fry appeared at the Albery Theatre for only three nights, last Thursday, Friday and Saturday. His understudy, Mark Anderson, had to take over the role on Monday when a note in the programme described Fry as "indisposed". Reviews of the play were generally good.
Fry's last public appearance was on Sunday night, when he narrated Peter and the Wolf during a charity concert at St Mary's Church in north London.
A member of the orchestra said: "He didn't look ill, but he didn't mix with the rest of the orchestra and he didn't go to the party afterwards."
The 37-year-old actor's withdrawal has thrown the production of Cell Mates, a comedy drama in which Fry played the Russian spy George Blake opposite Mayall as Sean Bourke, an Irish petty criminal, into confusion. Fry has driven himself extremely hard since leaving Cambridge. His curriculum vitae lists 43 plays, films, television programmes and serials in which he has appeared.