According to a statement, Johnston became ill last Wednesday with an erupted duodenal ulcer which had to be operated on immediately. Johnston - who plays the role of Joan and is best known for her appearances on television's “ER” and “3rd Rock From The Sun” — is hoping to return to the New Ambassador Theatre’s stage before Christmas.

In the meantime Molly Regan, who created the role of Joan in the play’s world premiere at Chicago’s Steppenwolf last April, has flown to London at a day’s notice to take over from Johnston.

In John Crowley’s production, which opened Dec. 4, Regan joins a starry cast featuring Neve Campbell, Michael McKean and Cillian Murphy.

Molly Regan has been a member of the Steppenwolf ensemble since 1985, appearing in many productions including Maria Arndt, David Copperfield, The Glass Menagerie, Time of My Life, Another Time (Jeff Award), Earthly Possessions, Three Sisters, Aunt Dan and Lemon, Miss Julie, Cat on a Hot Tin Roof, You Can’t Take It with You and Reckless. On Broadway she appeared in Stepping Out and The Crucible.

For more on Love Song, which is booking until March 3, call (0)970 0606627.