A columnist for the Reuters Breakingviews comment website has resigned while multiple breaches of its code of conduct on disclosure of financial interests and cases involving other commentators are being investigated, Thomson Reuters admitted today.

Neil Collins has resigned after it emerged he had failed to declare that he owned shares in a number of companies he was writing about, including BP, Marks & Spencer, Yell and Diageo. Thomson Reuters said it had launched an internal investigation, but there was no evidence Collins was "abusing his position for financial gain".

Reuters today put out a financial newswire story about the columnist's resignation and the ongoing investigation. It also refiled a number of columns by Collins and two other Breakingviews journalists, Neil Unmack and Margaret Doyle, going back to February last year, "to add disclaimer for author's personal investment". Unmack and Doyle remain in their jobs, Thomson Reuters confirmed.

The Reuters code of conduct forbids journalists from writing about shares they own unless they notify their interest to their manager and from dealing in shares about which they have written recently or intend to write in the near future.

David Schlesinger, Reuters' editor-in-chief, told staff in an internal note today that several other cases had come to light as a result of questioning Reuters Breakingviews staff where disclosures to readers or managers could or should have been made.

"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," said Schlesinger.

He added that Collins wrote commentary about companies in which he had a financial interest and made trades shortly afterwards. The investigation is continuing, Scheslinger added.

In an email sent by Collins to Hugo Dixon, the co-founder and global editor of Reuters Breakingviews, the financial commentator admitted that he is "saddened and embarassed" at the code of conduct breaches, but insisted he had not abused his position.

Reuters said it had added these disclaimers to a total of 53 online columns by the three journalists dating back to February 2009 – the majority written by Collins.

The disclaimer has been added to all relevant archived columns to say whether the commentators held shares in the companies at the time and if they traded them shortly before or after publication.

A total of 36 of the columns were published in 2009, while 17 were published this year.

From March 2009, Collins commentated on the financial affairs of 15 companies in which he owned shares. It is not clear for how long Collins owned shares in the companies mentioned before they were published.

The most recent column, published on 29 September and written by Collins, is on the BP executive board reshuffle after the Gulf oil disaster. Shares in the company reached a three-month high in the days following the news.

Unmack owned shares in Lloyds TSB contingent convertible bonds – dubbed "CoCos" – from mid-December 2009 up until mid-July this year and during that period he wrote about the company seven times. Doyle owned shares in RBS and the Bank of Ireland when writing about the companies.

Collins said in his email to Dixon, seen by MediaGuardian.co.uk, that he dealt shares "infrequently" and that he "saw an opportunity" in BP as the price fell in the aftermath of the Gulf oil spill.

"I failed to connect my comments for Reuters – among millions of words written on BP at the time – with the purchases," Collins writes. "I had read Reuters' rules about share dealing on joining the company in March 2009, but my recollection was incorrect. I did recall the phrase 'The test is whether the editorial activity might continue to have an impact on the securities.' This is surely the point of the rules."

Collins said he emailed Schlesinger on 4 October after a conversation with his fellow Breakingviews columnist Chris Hughes, who suggested he may be in breach of the code of conduct. The senior columnist admitted to failing to diclose his ownership of shares in companies he had written about, but said he did not trade them within Reuters' 30-day "exclusion zone".

He also admitted that he had sold a "substantial" stake in Marks & Spencer, obtained from his late father's estate, just five days after commenting on the company's financial results.

"As with BP, I view this as a serious, but technical breach of the rules. At no stage do I consider that I have abused my position at Reuters," Collins said

"On discovering the second breach, I felt I had no choice but to offer my resignation, which you accepted on 15 October... I am saddened and embarrassed by my breaches of the rules and hope that you will shortly be able to draw a line under this unfortunate episode."

Collins is one of the UK's most experienced and well respected financial journalists. He had already been City editor of the London Evening Standard and Sunday Times when he was hired in 1986 by Max Hastings, then the Daily Telegraph editor, to run the paper's business desk. He was City editor there for 19 years before leaving in 2005, just before the paper's business desk moved from Moorgate to Canary Wharf. It is a Fleet Street record that is unlikely to be broken. His column was essential reading in the Square Mile, renowned for its authoritative and witty take on issues including executive pay.

He went on to write a twice-weekly column for the London Evening Standard. Known for his acerbic style, he was named financial journalist of the year at the British Press Awards in 2002. He joined Thomson Reuters as a columnist in early 2009, moving to Breakingviews when the financial commentary website was bought by the news agency last year.

Doyle is a former Daily Telegraph and Economist journalist, specialising in investment banking. Unmack writes about credit markets, hedge funds and Italy for Reuters Breakingviews and previously worked for Bloomberg News and Euromoney Institutional Investor.

Thomson Reuters said it was reviewing procedures and training and had called on all Reuters staff to review their involvement in the financial markets to make sure they were complying with the code.

It is not currently a standard requirement that Thomson Reuters columnists and news staff declare any relevant securities holdings at the foot of columns and stories, but this policy is now being reviewed.

Thomson Reuters bought Breakingviews for about £13m in October last year and combined the business with its own team of financial comment writers. Breakingviews was founded by the former Financial Times journalists Hugo Dixon and Jonathan Ford in 1999.

Dixon took the job of global editor, Reuters Breakingviews, following the deal. Ford left Breakingviews in 2007 and is now chief leader writer for the FT.

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