The worlds of Casualty and Holby City will collide as the hospital comes under cyber attack in a special crossover episode.

The casts of the medical dramas will join forces for a two-part special on BBC One for the first time in 14 years.

Amanda Mealing, playing Connie Beauchamp, will be reunited with the cast of her old stomping ground Holby City during the episode, which comes as part of the programme’s 20th anniversary celebrations.

Elle and Sacha in the Casualty and Holby City crossover episode (BBC/PA)

The special will see Holby City Hospital plunged into a blackout, disabling much of its equipment.

In the episode, Sacha (Bob Barrett) is involved in a car crash, while in the hospital Connie will be forced to operate on a member of her team.

Jac (Rosie Marcel) comes to the rescue and is ready to operate but there is only one theatre available in the blackout.

Connie and Jac have to choose which of their friends to try to save first.

Both programmes are set in Holby City Hospital in the fictional city of Holby – Holby City was originally conceived as a spin-off of Casualty.

The former is set in the hospital’s acute assessment/admissions unit, general surgical ward and cardiothoracic ward while the latter focuses on its accident and emergency department.

Executive producer Simon Harper said: “We’ve known for some time there’s been major audience appetite for a mega crossover between these sister shows, so that’s exactly what we are serving up.

“You’ll see both the Casualty and Holby teams striving heroically against the odds in two episodes of pure, nailbiting, taut, emotional medical drama.

“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.”

Part one of the special will air during Casualty on March 2 while part two will air during Holby City on March 5 on BBC One.