The actor, who plays PC Roger Valentine, was taken ill during a break in filming on Sunday.

John, married and in his 40s, was diagnosed with cardiac arrhythmia — the irregular heartbeat condition that hit Tony Blair and can be life-threatening.

He was signed off work for two weeks and scripts had to be rewritten.

An insider at the ITV show said: “Everyone is incredibly worried.”

We wish John a speedy recovery.