Bradley told the Commons that a report by regulator Ofcom raised concerns over Murdoch family having “increased influence” over the UK’s news agenda and the political process.
“On the basis of Ofcom’s assessment, I confirm that I am minded to refer to a phase two investigation on the grounds of media plurality,” she said.
Bradley said Ofcom’s report was “unambiguous”, telling MPs: “The reasoning and evidence on which Ofcom’s recommendation is based are persuasive.”
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She added: “The proposed entity would have the third-largest total reach of any news provider – lower only than the BBC and ITN – and would, uniquely, span news coverage on television, radio, in newspapers and online.
“Ofcom’s report states that the proposed transaction would give the Murdoch Family Trust material influence over news providers with a significant presence across all key platforms.
“This potentially raises public interest concerns because, in Ofcom’s view, the transaction may increase members of the Murdoch Family Trust’s ability to influence the overall news agenda and their ability to influence the political process and it may also result in the perception of increased influence.
“These are clear grounds whereby a referral to a phase two investigation is warranted – so that is what I am minded to do.”
Bradley said parties involved in the process can make representations to her before she reaches a final decision, with a deadline of 14 July.
She added she is “minded not to refer” the bid to a phase two investigation in relation to a “genuine commitment to broadcasting standards”.
“While there are strong feelings among both supporters and opponents of this merger, in this quasi-judicial process, my decisions can only be influenced by facts, not opinions – and by the quality of evidence, not who shouts the loudest.”
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Shadow culture secretary Tom Watson said the decision was drawn from “the old playbook” as he predicted the government would eventually allow the merger to go ahead.
He said: “The secretary of state has known all along what she wants to end up doing but she has to follow the established dance steps.
“So let me make a prediction now. The parties have proposed some pretty minor undertakings in lieu.
“They always knew they weren’t going to be enough to satisfy Ofcom so the secretary of state will demand extra conditions, as a result of which she will be written up as a tough operator.
“The parties will offer something new which they always had in their back pockets, the secretary of state will accept them as they always planned and this merger will go ahead.”
Watson said undertakings from the Murdoch family were “not worth the newsprint they are written on” as he warned that lessons had not been learned from the phone-hacking scandal.
He told MPs: “Given everything we know about his [Mr Murdoch] and his company’s behaviour over phone-hacking, and given everything we know about Fox’s behaviour on the ongoing sexual harassment scandal in the United States, then that says more about the rules than it does about Mr Murdoch.
“It’s clear that the rules need to be reviewed and if the current Conservative government won’t do that then the next Labour government will.”
Watson vowed to “put media barons on notice”, saying it was time to end the culture of people dominating the UK media market but paying taxes overseas.
He also accused the Conservatives of forming “an implicit bargain” with the Murdochs as he pushed Bradley to order part two of the Leveson Inquiry into phone-hacking.
Bradley hit back at Watson’s claims, saying she was disappointed at “cynical” attempts to politicise the issue and to prejudge the decision.