Prime Minister David Cameron on Tuesday condemned as "truly dreadful" allegations that a British tabloid hacked the voicemail of a missing teenage girl who was later found murdered.
He said police should investigate the claims about the News of the World, Britain's top-selling Sunday title owned by Rupert Murdoch's News International, in "the most vigorous way they can".
However, Cameron made clear that the allegations about Milly Dowler, who disappeared aged 13 in 2002, would not prompt him to intervene in the bid by Murdoch's News Corp. to take a majority stake in satellite broadcaster BSkyB.
News International chief executive Rebekah Brooks said she was "sickened" by the claims and vowed to "vigorously pursue the truth".
Brooks is facing calls to quit because she was editor of the News of the World when Dowler went missing, but she said: "I am determined to lead the company to ensure we do the right thing and resolve these serious issues."
The tabloid has been dogged by claims of phone hacking ever since its royal editor and a private investigator were jailed for the practice in 2007, and a new investigation was launched in January amid a wave of fresh allegations.
But most of the targets had been celebrities and politicians, and news that the paper may have hacked into Dowler's phone has caused widespread disgust.
The teenager disappeared on her way home from school near London nine years ago, sparking a national campaign to find her. Her bones were found six months later in a forest. Nightclub bouncer Levi Bellfield was convicted of her murder last month.
According to the Guardian, private investigators and journalists listened to increasingly desperate messages on Dowler's phone left by her parents and friends as the days went by without any word from her.
When her voicemail box became full, they even deleted several messages to make room for new ones -- an action that her loved ones and police mistakenly took as proof that Dowler was still alive and using her phone, the report said.
"If they are true, this is a truly dreadful act and a truly dreadful situation. What I've read in the papers is quite, quite shocking," Cameron told a press conference during a visit to Afghanistan.
He said police "should feel they should investigate this without any fear, without any favour, without any worry about where the evidence should lead them. They should pursue this in the most vigorous way that they can."
However, Cameron said he would not intervene in the government's deliberations over News Corp.'s highly controversial bid to buy the 61-percent of BSkyB it does not already own.
"The government, on these processes, is acting in a quasi-judicial way and it is quite right that the Secretary of State for Culture, Media and Sport (Jeremy Hunt) carries out his role in that manner without any interference from anyone else in the government," he said.
News International has admitted liability for hacking the phones of several celebrities, including actress Sienna Miller and football pundit Andy Gray.
And on Tuesday, Brooks said the group was working closely with police to investigate the new claims.
"I have to tell you that I am sickened that these events are alleged to have happened," she said in a message to staff, adding that she had written to Dowler's parents, Sally and Bob, promising to "vigorously pursue the truth".
She continued: "If the allegations are proved to be true then I can promise the strongest possible action will be taken as this company will not tolerate such disgraceful behaviour.
"I hope that you all realise it is inconceivable that I knew or worse, sanctioned these appalling allegations."
The Dowlers' lawyer, Mark Lewis, earlier said they would be suing the tabloid for its "heinous" and "despicable" behaviour and said Brooks should "do the honourable thing".
Opposition Labour leader Ed Miliband called for a public inquiry, saying: "She should consider her position but this goes well beyond one individual."