A Calgary police constable refused to allow his mistress to be charged with drunk driving, according to his former fiancée who testified at the officer's trial Wednesday.

Mark McCullough, 44, is on trial for obstruction of justice and breach of trust, accused of helping Caren Lileikis escape an impaired driving charge.

In November 2015, Lileikis was found slumped over the steering wheel of her running car in a parking lot in the southeast community of Cranston and police believed she was drunk.

It wasn't until a year later, during a hostile break-up, when McCullough's former fiancée told police her ex had helped a woman get off an impaired driving charge, which kicked off an anti-corruption investigation into the officer's actions.

The call came into 911 dispatch as a "check on welfare" and was changed to an impaired driving code once officers arrived on scene, according to evidence called by prosecutor Leah Boyd.

But after McCullough got involved, it was again changed, this time re-categorized as a suspended driving code — a lesser offence — and the officer called off CPS breath technicians.

The 911 dispatcher who took the call happened to be McCullough's then-fiancée, Jessica Nelson, who testified on Wednesday.

McCullough told Nelson to call him so that their conversation wouldn't go over the radio for colleagues to hear.

"I'm not voicing anything over the air and I'm not putting anything on the system," McCullough said to Nelson.

In a conversation that was recorded from Nelson's desk line and played in court, McCullough then told his fiancée that he believed the suspected drunk driver was a key witness in "a big, big economic crimes investigation."

He said he was worried that if she was charged with impaired driving, it would affect her credibility at court as a witness. McCullough then told Nelson not to type any of that information into Lileikis's file.

Earlier in the day, Judge Frederick Fisher heard evidence from Det. Katherine Graham, who led the anti-corruption investigation into McCullough's actions. She testified there was no economic crimes investigation, rather the woman had witnessed a minor theft at a Home Depot months earlier and McCullough had taken a witness statement from her.

Nelson and McCullough dated from September 2013 to November 2016. They had a bitter break-up and custody battle.

McCullough was charged with assaulting Nelson at the end of their relationship but was acquitted after trial.

When Nelson went to police about the alleged domestic assault, she also disclosed that McCullough had once "let a friend off an impaired" charge.

Graham confirmed to defence lawyer Cory Wilson that despite looking for evidence of a relationship between McCullough and Lileikis, she found none from September 2015, when they met, to November 2015, when the officer is accused of obstructing justice.

Under cross-examination, Graham testified she was aware Nelson was "very angry" with McCullough when she went to police.

The detective also said CPS officers have discretion when deciding whether to lay a criminal or traffic charge.

In September 2016, toward the end of her relationship with McCullough, Nelson said she found Facebook messages that had been sent to him from women — including Lileikis — which concerned her.

A week after she confronted McCullough, Nelson said she received Facebook messages from Lileikis: "Did he tell you about the oral sex," the first one read.

Then Lileikis said she wanted to apologize for her relationship with Nelson's fiancée.

When Nelson confronted McCullough, he said the two were "just friends."

"I asked what else he'd done for her," said Nelson. "He said 'well there was that impaired, what are you supposed to do when you find a friend behind the wheel of a car? I had to take care of that.'"

Nelson said that McCullough eventually admitted he'd had a sexual relationship with Lileikis that began when they met after the Home Depot theft in September 2015.

But McCullough's lawyer questioned the timing of Nelson's disclosure to police. It was a year after the alleged drunk driving incident, during their toxic breakup, when she told investigators about it.

Nelson admitted she was angry with McCullough at the time, and Wilson suggested she was looking for a way to get back at him.

The trial is set for three days.