Britain’s spreading phone hacking and police bribery scandal forced two of London’s top police officers to resign in less than 24 hours and prompted Prime Minister David Cameron on Monday to call for an emergency session of parliament.

Scotland Yard chief Paul Stephenson stepped down on Sunday night, followed out the door on Monday by Assistant Commissioner John Yates. Mr. Yates was the official who decided two years ago not to reopen police inquiries into phone hacking and police bribery by tabloid journalists, saying he did not believe there was any new evidence to consider.

Detectives reopened the investigation earlier this year and now say they have the names of 3,700 potential victims.

The high-profile resignations have made it even harder for Mr. Cameron to contain the intensifying scandal that is threatening his leadership and knocking billions off of Rupert Murdoch’s global media empire.

Parliament was to break for the summer on Tuesday after members grilled Mr. Rupert Murdoch, his son James and Mr. Rupert Murdoch’s former British chief executive Rebekah Brooks in a highly anticipated public airing about the scandal. Mr. Cameron, however, wanted MPs to reconvene on Wednesday “so I can make a further statement.”

Mr. Cameron spoke in Pretoria, South Africa, on the first day of a two-day visit to Africa. He had planned a longer trip but cut it short as his government faces a growing number of questions about its cosy relationship with the Murdoch empire during a scandal that has taken down top police and media figures with breathtaking speed.

Opposition leader Ed Miliband said Mr. Cameron needed to answer “a whole series of questions” about his relationships with Ms. Brooks, Mr. James Murdoch and Andy Coulson, the former News of the World editor that Mr. Cameron later hired as his communications chief. Mr. Coulson resigned from that post in January and is one of 10 people who have been arrested in the scandal.

“At the moment, he seems unable to provide the leadership the country needs,” Mr. Miliband said.

Mr. Cameron insisted his Conservative-led government had “taken very decisive action” by setting up a judge-led inquiry into the wrongdoing at the now-defunct Murdoch tabloid News of the World and into the overall relations between British politicians, the media and police.

“We have helped to ensure a large and properly resourced police investigation that can get to the bottom of what happened, and wrongdoing, and we have pretty much demonstrated complete transparency in terms of media contact,” Mr. Cameron said.

Still, Mr. Cameron is under heavy pressure after the resignations of Mr. Stephenson and Mr. Yates, and Sunday’s arrest of Ms. Brooks — a friend of his — on suspicion of hacking and police bribery.

Ms. Brooks was detained and questioned for nine hours on Sunday before being released on bail. Her lawyer, Stephen Parkinson, released a defiant statement on Monday professing her innocence.

Mr. Parkinson said police would “have to give an account of their actions” considering “the enormous reputational damage” Ms. Brooks’ arrest had caused to the social and political insider.

Police are under pressure to explain why their original hacking investigation several years ago failed to find enough evidence to prosecute anyone other than News of the World royal reporter Clive Goodman and private investigator Glenn Mulcaire. Mr. Goodman has been re-arrested in the current hacking and police bribery investigation.

Mr. Stephenson, the police chief, resigned on Sunday over his ties to Neil Wallis, a former News of the World executive editor who has been arrested in the scandal. Stephenson said he had nothing to do with the earlier phone hacking inquiry or Mr. Wallis, but was resigning to allow his agency to focus on the London 2012 Olympics instead of leadership changes.

But in his resignation speech on Sunday, Mr. Stephenson made pointed reference to Mr. Cameron’s hiring of Mr. Coulson.

Mr. Cameron retorted that the situations of his government and the police were “completely different,” because allegations that police were bribed for information “have had a direct bearing on public confidence into the police inquiry into the News of the World and indeed into the police themselves.”

London mayor Boris Johnson said Mr. Yates had questions to answer about his own links with Mr. Wallis, and said Mr. Yates resigned after being told he would be suspended pending an ethics investigation.

Ms. Brooks’ arrest was the latest blow for Mr. Rupert Murdoch, the once all-powerful figure courted by British politicians of all stripes. Now Mr. Rupert Murdoch is struggling to tame the scandal, which has already destroyed the News of the World, cost the jobs of Ms. Brooks and Wall Street Journal publisher Les Hinton and sunk the media baron’s dream of taking full control of a lucrative satellite broadcaster, British Sky Broadcasting.

Mr. Rupert Murdoch is eager to stop the crisis from spreading to the United States, where many of his most lucrative assets — including the Fox TV network, 20th Century Fox film studio, The Wall Street Journal and the New York Post — are based.

Sky News reported on Monday that News Corp. had appointed a senior lawyer to head an internal probe on phone hacking.

Ms. Brooks was the bold chief executive of News International, Mr. Rupert Murdoch’s British newspaper arm, whose News of the World stands accused of hacking into the phones of celebrities, politicians and other journalists. But it was the revelation that journalists accessed the phone of slain teenager Milly Dowler in search of scoops while police were looking for the missing 13-year-old fuelled wide outrage in Britain.

At an appearance before U.K. legislators in 2003, Ms. Brooks admitted that News International had paid police for information. But she has always said she did not know any phone hacking was going on when she was editor of News of the World between 2000 and 2003 — a claim many have found difficult to believe.

Police have already arrested 10 people, including other former News of the World reporters and editors. One, Press Association royal reporter Laura Elston, was cleared by police on Monday. No one has been charged.

Even more senior figures could face arrest, including Mr. James Murdoch, chairman of BSkyB and chief executive of his father’s European and Asian operations. Mr. James Murdoch did not directly oversee the News of the World, but he approved payments to some of the paper’s most prominent hacking victims, including 700,000 pounds ($1.1 million) to Professional Footballers’ Association chief Gordon Taylor.

Mr. James Murdoch said last week that he “did not have a complete picture” when he approved the payouts.

At Tuesday’s committee hearing, which will be televised, U.K. politicians will seek more details about the scale of criminality at the News of the World. The Murdochs will try to avoid incriminating themselves or doing more harm to their business without misleading Parliament, which is a crime.

Mr. Hinton, too, could face questioning over wrongdoing at the News of the World during his 12 years as executive chairman of News International. But since he is an American citizen living in the U.S., British authorities would have to seek his extradition if he refused to come willingly.

Britain’s Serious Fraud Office, meanwhile, said on Monday it was giving “full consideration” to a request from a lawmaker that it open an investigation into Mr. Rupert Murdoch’s News Corp. activities in Britain.

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