Britain's top-selling Sunday tabloid vowed to take the ‘strongest possible action’ if it is proven that its journalists hacked the phone of a missing teenager who was later found murdered.

British Prime Minister David Cameron earlier said police should pursue their investigation into the claims about the News of the World, owned by Rupert Murdoch's News International, in ‘the most vigorous way’.

Cameron led a chorus of condemnation on Tuesday over allegations that the newspaper from Rupert Murdoch's global media empire hacked the voicemail of a missing schoolgirl who was later found murdered.

Suggestions that in 2002 a News of the World investigator listened in to, and deleted, messages left for the cellphone of the 13-year-old, misleading police and her family, caused uproar in parliament, where the tactics and power of the tabloid press, many of them Murdoch titles, have long caused controversy.

The gravest accusations yet drove the long-rumbling scandal into the heart of Murdoch's News Corp and forced Rebekah Brooks, a Murdoch confidante who was the News of the World editor at the time, to plead ignorance and say she would not resign as head of News Corp's British newspaper arm.

Pressure is unlikely to let up, however. A least one major advertiser, carmaker Ford, said it was pulling ads from the News of the World – though not the other Murdoch papers – until it saw how the tabloid dealt with the new allegation.

And police looking into phone hacking by the newspaper later said they had been in touch with the parents involved in another notorious child murder, when two 10-year-old girls were seized and killed by school caretaker in the town of Soham in 2002.

Suggestions that the News of the World's activities might have hampered police and given false hope to the family of the murdered teenager, Milly Dowler, caused uproar in Britain and moved Cameron to comment while on a visit to Afghanistan.

"On the question about the really appalling allegations about the telephone of Milly Dowler, if they are true, this is a truly dreadful act," Cameron told journalists in Kabul.

Lawmakers agreed to clear three hours of parliamentary time for an emergency debate on the issue on Wednesday. Cameron had until now said little about the phone hacking scandal, which forced the resignation earlier this year of his own spokesman, another former editor of the News of the World.

PREMIER'S SPOKESMAN

The News of the World's royal correspondent and a private investigator were jailed in 2007 after hacking another voicemail for scoops.

In January, angry celebrities, including entertainment and sports stars, as well as politicians, who feared their messages had been listened to prompted police to open a new inquiry.

Cameron, who is close to the Murdoch family and has often been spotted socialising with Brooks in Oxfordshire where they both have country homes, also reiterated that the BSkyB merger should be handled separately from the phone hacking probe.

Murdoch transformed the British press landscape in the 1980s during Margaret Thatcher's years as prime minister, bringing in new technology and confronting printers' and journalists' trade unions. He commands audiences with global leaders and, through his media, is seen as one of the world's most powerful men.

Analysts expect the $15 billion-plus BSkyB deal to go through despite political events.

"The secretary of state is not entitled to consider these latest appalling allegations when considering the News Corp proposal to buy the rest of BSkyB," said analyst Chris Goodall of media and telecoms research firm Enders Analysis.

The ministry reviewing the bid declined comment. But opposition Labour politician and former deputy prime minister John Prescott, who was told by police his phone may have been hacked, wrote to communications regulator Ofcom to ask it to review the bid and said it was not too late to halt the deal.

FALSE HOPE

The kidnap and murder of Milly Dowler is among the highest-profile criminal cases of recent years. Revelations from her family's lawyer that police were checking whether journalists manipulated her cell phone account while she was missing come less than two weeks after her killer was finally convicted.

Influential BBC journalist Robert Peston blogged on his Twitter feed: "News Int execs tell me they fear there may have been worse examples of NOTW hacking than that of Milly Dowler's phone. The mind reels." News International declined to comment.

Opposition Labour party leader Ed Miliband said News International Chief Executive Brooks, News Corp's most senior executive, should think about resigning.

But Brooks told staff in a memo she would not go. "It is almost too horrific to believe that a professional journalist or even a freelance inquiry agent working on behalf of a member of the News of the World staff could behave in this way," she wrote.

Facebook and Twitter campaigns sprang up in the wake of the latest allegation encouraging to boycott the News of the World.

News Corp long maintained that the cases were isolated and the work of a lone journalist gone off the rails. This year, it admitted liability in a few cases and will pay compensation to victims including actor Sienna Miller. Others, including actor Jude Law and soccer star Ryan Giggs are still suing the paper.

Peta Buscombe, chair of Britain's Press Complaints Commission, said News Corp had lied, leading the PCC to conclude in 2009 that there was no evidence the phone hacking was the work of more than one rogue reporter: "I personally and the PCC are so angry because clearly we were misled," she told the BBC.

Police, who have been accused of being sluggish in probing a media organisation to which some officers sold information, have arrested three journalists since re-launching their inquiries.