The identity of the mysterious character has not been revealed, with the ongoing secret continuing to grip that nation and propelling the series to become the most-watched show of 2019 so far.
Speaking at the BFI television festival, Mercurio – who created the show – said that, although there was a plan, he was always open to changing who “H” could eventually be.
”I suppose if I had an idea later on that was even better then, yeah, of course I would change it,” he said, “because you want it to be as good as possible and surprising and so forth.”
Mercurio, who also created Bodyguard, said he deliberately used names that could be turned into H, creating a series of red herrings in the show.
“Sometimes there were characters whose name could be corrupted or nicknamed into being an ‘H’ if I wanted to,” he said.
“Some of the characters were given names where they could have been revealed as having an ‘H’ connection if it was required.”
Viewers are hoping to find out the identity of H in the current series, with many speculating it could be Superintendent Ted Hastings – the head of anti-corruption unit AC-12.
The police drama, which first aired in 2012, has been commissioned for another series, but Mercurio admitted that he has not started writing it yet.
“We are not going into straight into series six, we don’t shoot them back to back,” he said.
“What we have always done, even though we’ve had two series commissioned, three and four, four and five, it’s helpful to the process that we are allowed time for the series to go out.
“Then we can take stock of audience reaction and that’s really helpful to us, the stories have resonated.
“Those help us plan ahead but we’re going to take stock after series five.”
Additional reporting by agencies.