“Our reporters should not have access to any data considered proprietary. I am sorry that they did,” wrote US editor-in-chief Matthew Winkler in an article that appeared on the front page of Bloomberg’s news service.

“The error is inexcusable,” he added in the 600-word mea culpa, entitled “Holding Ourselves Accountable”.

Winkler’s apology came as US regulators reportedly consider whether to probe Bloomberg’s behaviour in a major reputational crisis for the news giant.

Banks including Goldman Sachs and JPMorgan are said to have raised concerns after it appeared that reporters had used internal information.

During last year’s trading scandal at JPMorgan in London — involving a trader dubbed “the London Whale” and billions of dollars of losses — a journalist was said to have tried to find out whether disciplinary action had been taken.

Winkler insisted reporters had had only limited access to customer data and claimed this got “lost in much of this weekend’s conversation” after the scandal broke.

He said: “At no time did reporters have access to trading, portfolio, blotter, monitor or other related systems. Nor did they have access to clients’ messages to one another. They couldn’t see the stories that clients were reading.”

But he admitted that reporters could look at a users’ login history and see “high-level types of user functions on an aggregated basis” — and information about help-desk inquiries when a client contacts Bloomberg and asks for help on how to get a specific piece of information.

Bloomberg has now barred reporters from such access.

Privately owned by New York Mayor Michael Bloomberg, the company supplies news and data to around 310,000 clients at a cost of $20,000 (£13,000) a year per terminal.