Police are investigating allegations of email hacking at The Times.
Labour MP Tom Watson has received a letter from Scotland Yard informing him that the newspaper is being investigated after admitting that one of its reporters tried to access a private account.
The Times named Lancashire detective Richard Horton as the author of the award-winning NightJack blog in June 2009 after the High Court refused to grant him anonymity.
James Harding, the paper's editor, told the Leveson inquiry last month that one of his reporters - named as Patrick Foster - was issued with a formal written warning for professional misconduct for gaining unauthorised access to Mr Horton's email account.
In a further letter to the inquiry, Mr Harding said: "When the reporter informed his managers that, in the course of his investigation, he had on his own initiative sought unauthorised access to an email account, he was told that if he wanted to pursue the story he had to use legitimate means to do so.
"He did, identifying the person at the heart of the story using his own sources and information publicly available on the internet.
"On that basis we made the case in the High Court that the newspaper should be allowed to publish in the public interest. After the judge ruled that we could publish in the public interest, we did."
Mr Foster has since left the newspaper, Mr Harding said.
In a letter to the Metropolitan Police last month, Mr Watson - a long-term campaigner on the hacking issue - urged officers to examine whether a crime had been committed and whether the court had been given the full facts.
"In an attempt to protect his privacy, the police officer in question sought an injunction," the MP wrote. "Far from putting forward a public interest defence, lawyers representing The Times claimed that the information was obtained through entirely legitimate means...
"I must ask that you investigate computer hacking at The Times."
A letter from Detective Superintendent John Levett to Mr Watson, published on the MP's website, said: "I write to reassure you that the concerns raised within your letters are under investigation and officers from Operation Tuleta are dealing directly with the victim."
A Met spokesman said: "We can confirm that a letter was received on Monday January 23 from MP Tom Watson.
"Officers from Operation Tuleta are in contact with Mr Watson in relation to specific issues he wishes to raise and we are not prepared to give a running commentary on the Tuleta investigation."
A spokeswoman for The Times declined to comment.