Jed Mercurio says Line of Duty could run for another two series

The show has traditionally taken two years between each series, and series three, four and five were made in the knowledge that they would definitely not be the show's final run - allowing Mercurio to set up plot points for future series.

Series six is already commissioned, and Mercurio, speaking at the BFI and Radio Times Television Festival, said that he will discuss plans for series seven with the BBC once the dust has settled on the current run.

"Once series five has finished airing we'll have a conversation with the BBC, with a view to whether there's a possibility to series seven being commissioned," he said. "But we definitely know we've got series six."

This has always been the plan, it seems.

"The only reason is that there are a lot of loose ends to tie up, and we couldn’t take the risk of getting it commissioned later on," he said. "So if we didn’t know that we could go beyond this series, then we would have to approach it in a way that concluded things."

It's this crafty forward planning that is ultimately responsible for the extended delays between series, and that is likely to be no different this time around.

"We are not going straight into series six, we never shoot them back to back," Mercurio said.

"It's really helpful to the process that we are allowed time for the series to go out, and then we can kind of take stock of audience reaction. That's just really helpful to us – the stories that have resonated, the things that people are interested in – those help us plan ahead. So we're going to take stock after series five, and that's always the way that we do it."

Mercurio confirmed he hasn't started writing the upcoming series, but he has a clear idea of the direction it will take, based upon what happens in the final episodes of the current series.

"There are certain things that we know we have to pick up, in terms of the over-arching story, but work wouldn't even start until we decided what the production schedule was. That is something that has to be determined with the broadcaster and also cast availability, location availability – all those things end up being part of quite a complicated analysis."

But does he have an idea of where the show will end?

"I know a few key points, yeah," he concluded cryptically.

Line of Duty series five continues on BBC1 on Sundays at 9pm