The casts of BBC One’s Casualty and Holby City are to appear together for the first time since 2005.
They will appear in a two-part special when a cyber-attack plunges Holby City Hospital into chaos.
The storylines will be “intertwined” as characters from both series interact across the two episodes, the BBC said.
The crossover will begin during Casualty on Saturday 2 March and will conclude during Holby City on Tuesday 5 March on BBC One.
It is part of Holby City’s 20th anniversary celebrations, where throughout the year there will be guest appearances from returning Holby characters.
Amanda Mealing will return in the crossover episodes in her role as Connie Beauchamp.
The various storylines will include a car crash, a possible pregnancy and two members of staff will find their life in the balance, with only one theatre available in the blackout.
Simon Harper, executive producer of Holby City and Casualty said there had been fan demand for the episodes.
“We’ve known for some time there’s been major audience appetite for a mega crossover between these sister shows,” he said.
“You’ll see both the Casualty and Holby teams striving heroically against the odds in two episodes of pure, nail-biting, taut, emotional medical drama.”
He praised the production teams for their skills in combining the two shows which are filmed in Cardiff and Elstree.
“There’s so much to relish in the cross-show character interactions: an especially poignant thematic resonance between two regulars, Connie and Jac, in delicious conflict, and perhaps most vitally, who from Casualty has been secretly carrying on with whom from Holby!”