The culture secretary, Jeremy Hunt, is writing to Ofcom and the Office of Fair Trading seeking advice over News Corporation's bid for BSkyB in the light of phone-hacking revelations and the closure of the News of the World.

The news came as BSkyB shares fell to £7 in early trading, down 6.7% and back to the level of the original News Corp bid.

Hunt will ask Ofcom and the OFT, which have been advising him on the bid, whether the spate of revelations that led last week to the closure of the News of the World meant they would take a different view on the deal – and if so, whether they believed the Competition Commission should examine it.

Hunt would ask them to look at three aspects, the BBC said, most notably whether News Corporation's guarantees made in January about the TV network's independence and future could be trusted in view of the new phone-hacking developments.

Additionally, Hunt will ask Ofcom, the media regulator, if the company remains a "fit and proper" owner of BSkyB and whether the closure of the News of the World means the media plurality landscape needs re-examination.

The move could be a huge blow to Rupert Murdoch's media empire, suggesting that even the shock closure of the newspaper at the centre of the hacking scandal has not been sufficient to safeguard the BSkyB deal.

The bid also looks in potential jeopardy from a Labour Commons motion on Wednesday calling for it to be delayed. The motion looks set to receive strong Liberal Democrat support.

The Lib Dem party president, MP Tim Farron, told BBC Radio 4: "If a legally worded motion comes to the House opposing a further Murdoch takeover of BSkyB I can't see how Liberal Democrats would vote against that."

The decision by Hunt to write to Ofcom is the first sign that the government is looking for a way to block the takeover. It will also give the government a defensible political position on Wednesday's Commons vote over whether the bid should be blocked, although the legal status of such a vote is unclear.

The Conservatives do not want to be seen defending Murdoch in the current climate.

Hunt has two legitimate reasons to ask Ofcom to re-examine the status of the bid for BSkyB. The disappearance of the News of the World arguably changes News International's share of the UK newspaper market. In practice most people expect Murdoch to produce a Sunday tabloid in the shape of a Sunday Sun. At the least a new uncertainty has been injected into the equation.

Secondly, and potentially more damaging to News International, Hunt is to ask whether the assurances given by Murdoch about the editorial independence of Sky News need to be viewed in a new light given that senior NI figures appear to have been dishonest in their answers to a parliamentary select committee, the police and the Press Complaints Commission, as well as to the wider public.

That accusation has now been levelled by the chairman of the culture select committee, John Whittingdale, the Metropolitan police's assistant commissioner, John Yates, and the chair of the PCC, Lady Buscombe.

In that context it is legitimate for Hunt to ask whether this is an organisation that is fit and proper to hold a broadcasting licence.

In practice the OFT is incapable of giving a clear answer to the Hunt letter immediately, but it allows Hunt to say in Wednesday's debate that he realises wide long-term issues have now been raised and the issue is no longer one of narrow plurality.

John Whittingdale, the Tory chair of the culture select committee, joined calls for the Murdoch's BSkyB takeover bid to be put on hold.

He told the Today programme that given the "poisonous" atmosphere it would be difficult for the takeover to proceed.

"The best thing would be if it could be put on hold until we have a much clearer idea of who knew what, who was responsible," Whittingdale said.

"It could be a long time but I'm not sure it's in News International or News Corp's interest to proceed in the present climate. Now whether or not they will be considering putting it on hold or whether or not there is a legal mechanism – that's something I assume the OFT and Ofcom will advise – remains to be seen.

"There will be a question mark over whether or not the undertakings given can be trusted. All the events we have had today relate to News International, which is the British arm of News Corp. It is not NI which is taking over BSkyB. There would have to be question marks over directors of News Corp. And that essentially is the Murdochs."

Pressed on whether he would recall Murdoch deputy, Les Hinton, who previously gave evidence to the select committee and insisted at the time that the phone hacking was down to one rogue reporter, Whittingdale said the committee had yet to meet and discuss the latest developments. But he said matters were complicated by an ongoing police inquiry.

"There is a danger, as David Cameron has referred to in relation to the judicial inquiry, that the police inquiry has to take precedence. And then there is to be this judicial review, so there's going to be no shortage of people looking at this.

"And whether or not we were misled, it is a very serious matter to mislead parliament, but nevertheless it slightly pales in comparison to some of the other charges."

Nick Clegg, the deputy prime minister, has met with the family of murder victim Millie Dowler following the shock revelation last week that her phone was hacked. The family are due to meet the Labour leader, Ed Miliband, on Tuesday, with a meeting scheduled with the prime minister on Wednesday.