In the early 1950s he was cast in small parts in productions headed by John Gielgud and Noël Coward, and achieved increasingly prominent roles over the next decade.

His most enduring role was that of Agatha Christie's Belgian detective, Hercule Poirot, in a long sequence of radio adaptations of her novels, beginning in 1987 and continuing at intervals until 2007.

[1] At the same theatre played the sinister waiter in Anouilh's Point of Departure, with Dirk Bogarde,[6] making his West End debut when the production transferred to the Duke of York's.

[1] With the English Stage Company at the Royal Court he appeared in Nigel Dennis's Cards of Identity and Brecht's The Good Woman of Szechuan and attracted considerable attention as Mr Sparkish in Wycherley's The Country Wife.

[2] In September 1959 Moffatt joined the Old Vic company, playing in As You Like It, Richard II, Saint Joan, The Merry Wives of Windsor, Henry V and Barrie's What Every Woman Knows.

[1] In 1962 he won the Clarence Derwent award as best supporting actor of the past season for his portrayal of Cardinal Cajetan in John Osborne's Play at the Royal Court, transferring to the West End and Broadway.

His roles included Fainall in The Way of the World, Judge Brack in Hedda Gabler with Maggie Smith and Robert Stephens, directed by Ingmar Bergman, Menenius in Coriolanus, Cardinal Arragon in The White Devil, a range of parts in The Captain of Köpenick and Sir Joshua Rat in Adrian Mitchell's Tyger.

[2][3] In 1972 Moffatt was narrator and one of the main performers in the revue Cowardy Custard at the Mermaid, a compilation of the words and music of Noël Coward, who was present at the premiere.

[2] In The Bed Before Yesterday by Ben Travers (1975), Moffatt gave what The Times considered one of his subtlest performances as the hen-pecked husband opposite the sexually rampaging Joan Plowright.

In The Observer, Robert Cushman wrote, "John Moffatt, a master of the languishing comic art of flicking off a line without ever losing it, may be giving the performance of his life.

[9] In Ronald Harwood's Interpreters (1985) Moffatt played a Foreign Office official striving to keep the peace between Maggie Smith's Nadia and Edward Fox's Viktor.

His later radio roles included Oswald to Gielgud's King Lear, Lord Chief Justice to Timothy West's Falstaff and Quilp in The Old Curiosity Shop.

[2] His most conspicuous radio role was Hercule Poirot in 25 adaptations of Agatha Christie's detective stories, beginning with The Murder of Roger Ackroyd on 24 December 1987.

He played Joseph Surface in The School For Scandal, Brush in The Clandestine Marriage, the Prince of Aragon in The Merchant of Venice, Casca in Julius Caesar, Malvolio and Sir Andrew in two different productions of Twelfth Night, and Ben in R.F.