The parents of Holly Wells and Jessica Chapman, the two children murdered by Ian Huntley, were contacted by Scotland Yard detectives investigating phone hacking at the News of the World, it emerged on Tuesday.

A spokesman for Cambridgeshire police said they were aware that the families of Wells and Chapman were contacted by the Metropolitan police about two months ago.

It is believed the families were warned there was evidence to suggest they were targeted by Glenn Mulcaire, the private investigator who was formerly employed by the paper.

The families are thought to be seeking further clarification from the Met but are not currently commenting. Scotland Yard is conducting an investigation, Operating Weeting, into the News of the World phone-hacking allegations.

Pressure is growing on Rebekah Brooks, who was editing the News of the World at the time and is now chief executive of its parent company News International, following the Guardian's revelations on Monday that Mulcaire hacked into a mobile phone belonging to Milly Dowler in 2002, the same year as the Soham murders.

Schoolgirl Dowler went missing from her home in March 2002 and her body was found six months after she disappeared. The Soham murders took place in August that year.

Ed Miliband, the Labour leader, said earlier on Tuesday that Brooks should "consider her conscience and consider her position" as he called for a public inquiry into the hacking allegations and the conduct of the tabloid press as a whole.

Brooks insisted in an email to staff on Tuesday afternoon that she knew nothing about the allegations that Dowler's phone had been hacked by the paper she edited.

She said she was "sickened" by the events and added she was "determined to lead the company to ensure we do the right thing and resolve these serious issues".

News International executives insisted Brooks had the full backing of Rupert Murdoch, chairman and chief executive of News International's owner News Corporation.

Meanwhile, the chairwoman of the Press Complaints Commission, Baroness Peta Buscombe, which concluded in November 2009 there was "no new evidence" of widespread hacking at the News of the World following earlier Guardian revelations, admitted she had been "misled" by the News International paper.

Buscome said: "There's only so much we can do when people are lying to us. We know now that I was not being given the truth by the News of the World."

The home secretary, Theresa May, also added her voice to the growing chorus of politicians condemning the News of the World, including prime minister David Cameron.

May used stronger language than Cameron, who described the hacking of Dowler's phone as a "dreadful act" while visiting troops in Afghanistan. The home secretary told a committee of MPs on Tuesday afternoon: "I think it's totally shocking; frankly, it's disgusting. The mindset of somebody who thinks it's appropriate to do that is totally sick."

James Harding, the editor of the News of the World's News International stablemate the Times, also publicly condemned the behaviour, telling an audience of advertising executives that if "it [the Dowler allegation] is true, it seems to me what has happened is disgusting and indefensible and for us as journalists it is profoundly depressing".

Harding added: "My concern is, the shame is not just on the people involved, not just on that particular newspaper, but journalists in general."

The Labour MP, Tom Watson, said in the House of Commons earlier this year that the parents of the two girls killed by Huntley in Soham may have been hacked.

The public reaction to news that Dowler's phone was hacked has also been one of anger. Several hundred people have joined a Facebook site calling on readers to boycott the News of the World and some customers have cancelled their subscriptions to the Times and the Sunday Times, according to people close to News International.

The number of people cancelling is not thought to be high but it is regarded as symbolic internally. Members of the public have also been calling the News of the World's offices in London to complain about the paper's behaviour.

A senior News International executive said the atmosphere at the company was "subdued".