The government risked a bruising confrontation with media mogul Rupert Murdoch after it ordered Ofcom yesterday to investigate the public interest issues arising from British Sky Broadcasting Group's controversial move to buy a 17.9% stake in commercial broadcaster ITV last year.

In a test of the influence held by Murdoch, BSkyB's chairman and largest stockholder, over the media, Alistair Darling, the Trade and Industry Secretary, asked the media watchdog to conduct an initial inquiry into the public interest considerations of Sky's holding under the Enterprise Act (2002).

The secretary said he has asked Ofcom to report back to him by April 27.

Darling's decision follows heavy lobbying by MPs for an inquiry. A cross-party early-day motion was put before Parliament late last month.

The "public interest test" invoked by the government is designed to allow it to block mergers it believes could suppress competition in the media.

Darling said he wanted to "emphasise that this decision only means there will be an initial investigation by Ofcom", adding it was "without prejudice to any decisions I take subsequently on whether a fuller investigation by the Competition Commission may be necessary".

BSkyB said it would co-operate fully with each inquiry, but added that it believed its shareholding in ITV had no bearing on media ownership issues.

"It is inconceivable to suggest that, as a result of a 17.9% shareholding in ITV, Sky would be able to influence ITV's broadcasting strategy or policies, including programming or editorial decisions," the satelite broadcaster said.

The Office of Fair Trading announced in December it was also looking into the deal from a competition perspective. Both the OFT and the DTI have the power to instruct the Competition Commission to carry out a full inquiry.

City analysts said they were not surprised by Darling's move. Numis Securities said the DTI investigation into the deal was predictable, "given what a hot potato it is politically".

BSkyB, which is 39% owned by Murdoch's News Corporation, bought the ITV stake in November 2006 in a move that most analysts viewed as an attempt to prevent its pay-TV rival NTL from acquiring ITV.

NTL, which is now called Virgin Media after a merger with Richard Branson's Virgin Mobile, had announced shortly before BSkyB's move that it was in initial talks with ITV about an offer.

NTL and Branson complained bitterly about BSkyB's decision to buy a stake in ITV, but the OFT's investigation will not consider whether the satellite company's holding was intended to block NTL, but simply if BSkyB itself intends to take over ITV. BSkyB has denied any such ambitions.

In addition to the stake in BSkyB, News Corp's British subsidiary News International also owns the Times, Sunday Times, the Sun and the News of the World.

The Sun threw its suppport behind New Labour in 1997 after supporting the Conservatives for many years. Prime Minister Tony Blair and Chancellor Gordon Brown have met Murdoch several times since New Labour came to power and deem the support of his newspapers important.

Virgin Media and BSkyB are also locked in a separate dispute over the fees that BSkyB charges Virgin for its content.

BSkyB has warned that its operating profit could be hit by up to £20m if it failed to reach a deal to allow its channels to be broadcast on the rival Virgin Media service.

Talks broke down last week after Virgin Media accused BSkyB of "bullying" and "arrogance". Virgin said BSkyB was charging too much for the channels that broadcast top programmes including 24, Lost, and new episodes of The Simpsons.