But it was only when he was cast in BBC drama Undercover as Nick Johnson, a fictional version of one of those undercover officers, that Lester began to research the lengths the subterfuge went to and the pain it caused. The  police's official apology in November 2015, which was accompanied by financial settlements with seven victims, acknowledged that, "these relationships were a violation of the women's human rights, an abuse of police power and caused significant trauma", and Lester completely agrees.

"The methods that they used to infiltrate the groups, the needless methods they used to inveigle their way into the lives of members of those groups, and the way they disappeared, went beyond the bounds – and everyone including the police force agrees on this now – of decency," the 47-year-old actor says. "They overstepped the mark.

"If they were groups we thought were a clear and present danger to public safety, you could understand the police force using taxpayer money to go undercover and infiltrate them.

"But you're talking about people who were so low risk and so low key in how they protested, and it was the organisational skills and cash that the undercover officers had that helped these tiny groups take action."

As written by Peter Moffat (Cambridge Spies, The Village), Undercover's Johnson chose his burgeoning relationship with Maya Cobbina (Sophie Okonedo), a crusading young anti-racism activist and lawyer he monitored in 1990s London, over his job. He quit the force, stayed in character, married Maya, and fathered three now teenage children with her. It's Maya's rise from QC to potentially serving as the state-appointed director of public prosecutions that brings Nick's true identity into focus.

"Peter looked at it and knew it was really good territory for a drama," Lester says. "There's always a question mark when we let someone so closely into our lives, and the story plays on that."

Undercover is laced with official conspiracies and corrupted power structures, but the enthralling core of the six-part series is the relationship between Nick and Maya, which is expertly arranged by Lester and Okonedo. Their happy marriage is genuine, but the foundation it's built on is fraudulent, and the deception marks every seemingly positive exchange between them as the tension grows.

"It taps into one of our latent worries when we're in a long-term relationship – we're showing a couple who are very, very happy, yet she has no idea who her husband really is," Lester says. "It asks us to consider how well you really know the person you're with. You spend every day with them, but where did they go and what they really do before you knew them?"

Lester, who came to prominence in the 1998 Hollywood film, Primary Colors, and had a starring role in the long-running BBC con game melodrama, Hustle, had spent several years playing roles such as Othello in Shakespeare's The Merchant of Venice on West End and Broadway stages when he was cast in Undercover. What he wanted to capture back on television was how Nick, with the best of intentions, had nonetheless mired his family in deceit.

"It would be easy for him to be evil and let people boo because every time he's on screen you can see he's being horrible," Lester says. "But what Peter wrote was a character that I knew for the drama to work the audience had to be conflicted about.

"You believe that Nick is truly in love with this woman, and truly in love with his children. When you see him on screen, no matter what questionable things he does as the series goes on, I wanted people to absolutely believe that he was a good guy, and then let the drama unfold."

Undercover was well received in Britain, continuing the BBC's run of impressive dramas that included The Night Manager and London Spy, but Lester  says his Twitter account ran hot after the final episode aired as some viewers debated the ending while others protested  about a cliffhanger conclusion. A second series is by no means a certainty.

"I'm certain the conversations are being had, but whether you can follow what we did in the first series is uncertain," Lester says. "After all, it is called Undercover, and it's not clear if the characters can continue after so many revelations. This could be all there is."

WHAT Undercover

WHEN BBC First, Sunday, June 19, 8.30pm