LONDON » At a time when Western leaders are clamoring for greater powers to conduct covert surveillance, a prankster in Britain has turned the table on the watchers, securing a private cellphone number for a top intelligence chief and apparently making a separate phone call to the prime minister in his name, British officials acknowledged Monday.

The unidentified caller then phoned a tabloid newspaper Sunday to boast. He told the tabloid, The Sun, that he had been high on alcohol and drugs when he persuaded GCHQ, the British electronic surveillance agency, to give him a cellphone number for its director, Robert Hannigan.

Later, an unidentified caller widely believed to be the same person pretended to be Hannigan in a separate call to Prime Minister David Cameron.

"I’ve just made complete monkeys out of GCHQ; I’ve got the mobile number of the director," the caller, who was not identified by name, told the newspaper, referring to the Government Communications Headquarters agency, which collaborates closely with the National Security Agency in the United States. GCHQ also works with Britain’s domestic and overseas intelligence services.

"I’m definitely going to do it again," he said after calling GCHQ early Sunday, the newspaper reported. "It was so easy."

The Sun described the caller as well spoken and in his 20s, and it said that he had pretended to be a representative of Cameron’s office at 10 Downing Street when he called the switchboard at GCHQ. He was given a private cellphone number for Hannigan, which he used to call him. The spy chief became suspicious and hung up.

"Following two hoax calls to government departments, a notice has gone out to all departments to be on the alert for such calls," according to a government statement. "In the first instance, a call was made to GCHQ which resulted in the disclosure of a mobile phone number for the director."

"The mobile number provided is never used for calls involving classified information," according to the statement. "In the second instance, a hoax caller claiming to be the GCHQ director was connected to the prime minister."

"The prime minister ended the call when it became clear it was a hoax," it said.

In neither instance was delicate information disclosed, it added.

The intelligence agency and the prime minister’s office "take security seriously," according to the statement, "and both are currently reviewing procedures following these hoax calls to ensure that the government learns any lessons from this incident."

In remarks after a speech in Hampshire, Cameron said he became suspicious when the caller apologized for waking the prime minister, even though it was 11 a.m. and he was out for a walk, carrying his daughter Florence on his back.

"It claimed to be a conference call established, which I do obviously very frequently, between the head of GCHQ and some of the staff in my office," he said.

"A voice came through, a voice I didn’t recognize," Cameron said. "The voice said that he was sorry to wake me up, which I thought was strange as it was 11 o’clock in the morning, and so I quite rapidly asked ‘Who is this?’ to which the answer came ‘It is a hoax call,’ and so I pushed the red button on the BlackBerry, which ended the call."

"No harm was done, no national security was breached," Cameron said. "But it is important, when these things happen, to make sure we do everything we can to put in place in systems to weed out hoax calls. But every now and again I suspect these things will happen."

The episode came as Western spy chiefs are seeking greater powers to monitor social media sites and other Internet services to keep an eye on communications among terrorists after the attacks this month on a satirical newspaper, a police officer and a kosher supermarket in the Paris area.

The GCHQ website Monday carried what it called a news article saying that the agency "announces the reissue of ‘10 Steps to Cybersecurity,’ providing updated guidance on the practical steps that organizations can take to improve the security of their networks and the information carried on them."

Alan Cowell, New York Times