The arrest, the 18th since a renewed police operation began in January, suggested that investigators may have reached a crucial phase in a case that has raised disturbing questions about Britain’s freewheeling tabloid press and its relationships with the police, top politicians and even intelligence services.

In 2007, Mr. Mulcaire was jailed for hacking the phones of the British royal family at the behest of a reporter at Mr. Murdoch’s News of the World tabloid. The company insisted that the illegal acts were the work of the investigator and just “one rogue reporter.”

But in July, the scandal exploded after The Guardian disclosed that the tabloid had hacked into the phone of a murdered teenager in 2002, before her body was found. A wave of public outrage prompted three police investigations, a parliamentary panel and a public inquiry centering on accusations that the tabloid sought scoops by intercepting voice mails of newsworthy figures. How much top Murdoch executives knew, and when, remains a central question.

Investigators who have examined the 11,000 pages of Mr. Mulcaire’s meticulous notes on his work for the tabloid that were seized in 2006 say there is evidence that he had received 2,266 requests for interceptions from 28 journalists. The police have said that as many as 5,795 people may have been targeted between 2001 and 2009.

A statement from Scotland Yard said the 41-year-old man, who remained unidentified by the police on Wednesday, was arrested in London at 7 a.m. “on suspicion of conspiring to intercept voice mail messages” and of perverting the course of justice. Mr. Mulcaire’s lawyer, Sarah Webb, declined to comment when reached by telephone shortly after the reports identifying him as the suspect emerged.

The police and government inquiries have uncovered not only widespread phone hacking, but also police bribery and political connections all the way to the prime minister’s office. Two top police officers have been forced to resign. Mr. Murdoch and his son James closed the 168-year-old News of the World, were called before Parliament, and withdrew their $12 billion bid for a British satellite company under public pressure. Prime Minister David Cameron has been questioned over his hiring of a former News of the World editor, Andy Coulson, as his spokesman. Mr. Coulson is among those arrested.

So far, suspects have been released on bail after being questioned, but some, including Mr. Coulson, will face further questioning and perhaps charges.

The arrest came a week after a 31-year-old woman was detained in the same inquiry in the northeast of England. Scotland Yard declined to identify her, but the BBC and other British media identified her as Bethany Usher, a former News of the World reporter who is a lecturer in journalism at Teesside University. In a statement, she denied the hacking allegations.

“I worked for national newspapers between 2005 and 2008, spending two of those years at The News of the World, working largely on the road in the north of England,” Ms. Usher said. “At no time did I work in the Wapping office and I had little contact with other colleagues.” The newspaper’s head office was in Wapping.

“I have never been involved in the interception of telecommunications in any way and strictly adhered to the Press Complaints Commission code of practice,” she added, referring to the regulations of British journalism. “However, I became disillusioned through working with some who saw human suffering simply as fodder to fill pages. As such, I made the decision to find an alternative career.”

In late November, the police announced the arrest of a 52-year-old man, the first in a related investigation into computer hacking. He has not been identified by name, but was granted bail until December.

According to Britain’s Press Association news agency, Scotland Yard detectives are combing through 300 million e-mails from News International, a British media subsidiary of Mr. Murdoch’s News Corporation global empire.

Ravi Somaiya reported from London, and Alan Cowell from Paris.

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