Thomson Reuters Corp. (TRI) said Monday that columnist Neil Collins, one of the London's best-known financial journalists, had resigned after he breached the media and data information group's code of conduct on share dealing, and that other journalists are under investigation for similar dealings.

Collins, who was a columnist at Reuters Breakingviews, had written about several companies, including BP PLC (BP), Rio Tinto PLC (RIO.LN) and Marks & Spencer Group PLC (MKS.LN) in which he had a financial interest and made trades shortly after writing, David Schlesinger, editor-in-chief of Reuters, said in a note to staff.

Thomson Reuters, Dow Jones & Co., and some other financial media organizations have codes of conduct that restrict employees' investment and trading activities to avoid accusations of conflict of interest.

"While we have no evidence the journalist was abusing his position for financial gain, we take such breaches extremely seriously and that journalist resigned with immediate effect during our investigation," Schlesinger said.

Collins, who resigned last week, couldn't be reached for comment.

"Subsequent questioning of Reuters Breakingviews staff revealed several other cases where disclosures to readers or managers could or should have been made; we are updating the archive where appropriate and will continue to investigate these instances," Schlesinger said.

A Thomson Reuters spokeswoman declined to identify the other journalists, noting it was "too early" and the company is "bound by employee confidentiality."

A Reuters terminal Monday showed a list of republished columns with disclaimers that disclosed when a columnist had a significant interest or where the columnist dealt in the shares of the column's subject one month either side of when the article was written. The republished columns include articles not only by Collins but also columnists Neil Unmack and Margaret Doyle.

Unmack and Doyle weren't available for comment. The Thomson Reuters spokeswoman confirmed they still both work for the company.

Thomson Reuters bought Breakingviews in December 2009 with the aim of adding opinion to the company's traditional offering of news. The Wall Street Journal quoted people familiar with the matter saying the deal was valued ay between $15 million and $20 million.

The Thomson Reuters code of conduct says journalists shouldn't write about shares they own unless they notify their interest to their manager. Journalists also shouldn't trade in shares they've recently written about or intend to write about in the near future.

Schlesinger said the company would review its training to minimise the risk of breaches of the code of conduct happening again.

In his memo Schlesinger urged staff to look at their own market participation "to be sure you comply with the spirit and the letter of our rules."

"This is about our compact with our readers; it is about our individual reputations; and it is about ensuring that Reuters and Thomson Reuters live up to the standards set both by our long, proud history and our Trust Principles," he said in the note.

Dow Jones & Co., publisher of this newswire and The Wall Street Journal, competes with Thomson Reuters in supplying financial news.

According to his profile on Breakingviews, Collins has been a financial journalist in the U.K. since the 1970s, having been business editor of the London Evening Standard, The Sunday Times and then the The Daily Telegraph until 2005. He joined Reuters in March 2009. The profile says he is also a director of Templeton Emerging Markets Investment Trust PLC and Finsbury Growth & Income Trust PLC.

-By Lilly Vitorovich, Dow Jones Newswires; 44-0-207 842 9290; lilly.vitorovich@dowjones.com