LONDON — Tony Blair is the latest high-profile person to surface in the British phone-hacking trial, a high-stakes criminal prosecution of shadowy practices at Rupert Murdoch’s News of the World tabloid.

Blair, the former prime minister of Britain, offered to act as an “unofficial adviser” to Murdoch and to Rebekah Brooks, the former head of Murdoch’s British newspaper empire, who is one of eight defendants in the case and is expected to give evidence for the first time Thursday.

In a phone conversation less than a week before Brooks was arrested in July 2011, Blair told her to “keep strong” and take sleeping pills, according to an email in which she relayed the conversation to Murdoch’s son James. The email was read out by the prosecution in court Wednesday.

The email from Brooks to James Murdoch, dated July 11, 2011, said Blair “is available for you, K.R.M.” — shorthand for Murdoch senior — “and me as an unofficial adviser, but needs to be between us.”

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“It will pass,” Blair told her, according to the email, advising her to commission an independent inquiry. He also suggested the name of an outside lawyer, Ken Macdonald, “a great and good type,” the email read. On July 15, 2011, Brooks resigned from News International, the umbrella company covering the Murdoch newspapers in Britain, and on July 17, she was arrested.

Blair’s office issued a statement Wednesday saying he had been “simply giving informal advice over the phone.”

The trial has become one of the most high-profile criminal prosecutions in recent times in England.

The disclosure in 2011 that journalists at The News of the World had years earlier intercepted the voice-mail messages of a kidnapped teenager who was subsequently found slain caused widespread outrage in Britain. The case has since mushroomed, involving at least 1,000 likely victims from politics, sports, show business and the media. Murdoch shuttered the newspaper in 2011 as the scandal deepened.

Brooks, 45, is accused of condoning if not encouraging such practices among journalists, of bribing at least one official, and of conspiring with her husband to hide evidence. She has pleaded not guilty.