Pressure was mounting today on David Cameron over claims his aides had encouraged the rightwing media to publish smear stories against the Liberal Democrat leader, Nick Clegg.

Clegg's election coordinator, Danny Alexander, said it appeared that the Tory high command had orchestrated a campaign of negative stories in the run-up to last night's debate in an attempt to neutralise his surge in the polls.

The accusation came as polls revealed that neither Clegg nor Cameron could claim a clear win from last night's debate.

Provisional viewing figures showed 4 million people watched the debate across the three channels that carried it live: Sky News, Sky Three and the BBC News channel. A further 300,000 watched a repeat on BBC2.

Sky came under fire for stoking the anti-Clegg campaign, with more than 100 people complaining to the TV regulator, Ofcom, about the conduct of the debate moderator, Adam Boulton.

It is being alleged that Boulton, Sky's political editor, breached the strict rules governing the debate by raising with Clegg the front page of yesterday's DailyTelegraph.

The paper carried a story relating to donations to pay for a member of staff, which were paid into Clegg's personal bank account. The donations were all properly declared, and there has been no suggestion that the money was used for any other purpose. Clegg has denied doing anything wrong.

Alexander said: "I think that now what it looks like is that the first act that team Cameron took after his pledge to redouble the positive was to try and orchestrate a media smear campaign against us which had the effect of quadrupling the negative.

"George Osborne needs to come clean as to whether he himself was personally responsible for this negative media smear campaign, which is now backfiring spectacularly with voters."

He said the campaign reflected "a panic in the old political establishment that they are in a battle for survival".

"If Nick Clegg and the Lib Dems are able to change the way things work in this country in ways we have prepared to do, they see it as a huge threat," he added.

"The election now is between hope and fear and the vested interests are frightened the people might make their own choice."

Four newspapers ran attacks on Clegg on the eve of the debate, including claims that he was funnelling money from businessmen into his private bank account and that he had committed a "Nazi slur" against the British people.

Osborne met some political editors on Monday and discussed the party's response to the Lib Dem surge.

In one paper, a strategist was reported as expressing the hope that the media would do the Tory party's dirty work. There is no evidence that Osborne made this remark or that Conservative headquarters fed any story to any paper.

During a press conference convened to accuse Labour of "scaremongering" regarding the elderly in their campaign literature, the Tory chairman, Eric Pickles, and the shadow schools secretary, Michael Gove, were forced to defend the Conservatives from accusations they were involved in the onslaught of negative press against Clegg and his party.

"We've been insistent this week that we wanted to redouble the positive," Gove said. "That's one reason why David Cameron did so well [in the debate]. You'll see that the Daily Telegraph story came from their groundbreaking expenses investigation. If you look at the other stories, they related to things in the public domain. We have been responding to dealing with the electoral landscape as it is by emphasising the positive.

"All the stories which appeared were based on material which either the Telegraph has secured based on its own research, or information in the public domain – articles Nick Clegg has written, policies the Liberal Democrats have. This is the press operating in the way it always has, putting all politicians from all parties under scrutiny."

Pickles added: "We have a free and vigorous press, and beware of politicians who tells you that the press have gone too far. Beware of politicians who want to put restraints on the press. We don't complain when articles are written about us and we might be unhappy about it."

When challenged by one journalist on whether this was true, he said: "Well, we don't go too far – we might be occasionally a bit unhappy, but we don't think it's a great conspiracy.

"This is not Zimbabwe or the Soviet Union. No political party, no government controls the press.

"The Daily Mail, the Daily Telegraph, the Express, the Times, the Sun, the Financial Times ... all these newspapers are edited by grown-ups and read by grown-ups who make their own minds up."