The creator of Line of Duty already knows how the drama will end

Mercurio tells RadioTimes.com that he is keen for a sixth series in 2019, but that if it is not forthcoming he already has an idea of how the final episode will play out.

“If, by the time we get to the development of series five, there are no further series commissioned, then we have to approach series five as if it’s the last,” he said. “If we know we have got a sixth series then we are able to look ahead.

“I have got an idea of how things will conclude or, more accurately, the kind of story we need to get into for that to be dramatised," he added. "Whereas if I know series five is not the last series then I have other ideas that are more standalone in terms of what the story is.”

So does he have an idea for a final series in his head?

“Yes, the conclusion, yes. They are half ideas. I haven’t fully developed them. What will happen over the course of this year is that we will know whether we are doing more than just series five and everything will follow on from that.”

Asked how many series he would like to make if the decision was his, Mercurio replied: “I certainly would make six. Whether I would go beyond six would depend on how series five went.

“One of the advantages of commissioning two ahead is that it allows you to take a breath and look forward to the next one, and at the moment there is a great appetite among all of us involved in the programme to do more. What we don’t know is whether there will come a point where we kind of lose that enthusiasm to keep going back."

Asked if he is worried about matching the hugely successful and dramatic conclusion to series three – which saw the bringing to justice of Craig Parkinson's nefarious Matthew 'Dot' Cottan – he said: “That was a good while ago. You just want an audience to have a good feeling about a series, that it will deliver excitement and good drama and that’s what propels them into watching the next series.

“There’s a difference between a last episode and a first episode. One is trying to deliver answers and the other is trying to set up questions.

“We have got the fifth series and I do know that we have got massive backing from the broadcaster. I have been in situations where you are on a knife edge because there isn’t massive backing form the broadcaster and in those situations, you do get anxious because very few series are runaway hits. Most series perform in a middle ground."

Series 4 sees DS Kate Fleming (Vicky McClure) and DS Steve Arnott (Martin Compston) investigate Thandie Newton's DCI Roz Huntley who has been accused of tampering with evidence to convict a man with learning difficulties who is suspected of abducting women.

Line of Duty airs on BBC1 later this month