Beacon Hill-Cyrville Coun. Tim Tierney has admitted it was he who sent a newspaper journalist a confidential City of Ottawa memo regarding a lawsuit over the Rideau Street sinkhole, bringing a swift end to the integrity commissioner's investigation into the leak.

Tierney told council colleagues on Wednesday that he'd been "careless," but said he didn't share the information intentionally.

"I can commit to you that I'll be much, much more careful when emailing documents to others, and I'll never breach the council's confidentiality requirements again," he said.

The confidential memo was dated May 10, and described for staff and council members how the City of Ottawa was planning to sue its insurers for the Confederation Line project for $361 million over costs related to the 2016 sinkhole.

The Ottawa Citizen first reported the story on the evening of May 11, after the lawsuit was filed in court. CBC reported the details the following morning. Most of the information in the memo was contained in the publicly available statement of claim, but it was the first time the public learned of the significant extra costs associated with the incident.

Many council members were so concerned that the memo had been made public that on May 26, they voted 18-6 to ask the integrity commissioner to investigate, despite being advised the inquiry could cost $25,000 and may not yield results.

Tierney was among those councillors who called for an investigation.

"After council's request to the city's integrity commissioner on the leak, which I supported, I discovered I had inadvertently included the confidential memo in a bundle of documents that I had sent to a journalist on an unrelated public issue," Tierney explained Wednesday.

"That was a mistake, made in good faith. I am not making excuses whatsoever. The fact remains I sent the memo and I breached the code of conduct for members of council."

Mayor Jim Watson said Tierney had already told a member of his office on June 7, as well as the city clerk.

"Council colleagues, I'm of the view ... his apology and his suggested next steps have effectively ended the matter and made the investigation by the integrity commissioner redundant," Watson told council.

Asked by a journalist if Tierney could face some form of punishment, the mayor said no appropriate sanctions are available.

This is not Tierney's first apology this council term. After the 2018 election, Ontario Provincial Police charged him with corrupt practices under the Ontario Municipal Elections Act for trying to induce another candidate to drop out.

Those charges were withdrawn after Tierney apologized in court for making a "mistake" and forfeited two months' pay.