More than 100 viewers have complained to broadcasting regulator Ofcom that Nick Clegg was unfairly questioned during the Sky News leaders' debate on Thursday night.

They have accused Sky News political editor Adam Boulton, who moderated the debate, of breaking strict impartiality rules by raising newspaper reports about political donations paid directly into the Liberal Democrat leader's bank account. There is no suggestion Clegg broke any rules.

Ofcom has a legal duty to ensure all current affairs and political programmes are fair and balanced. A spokeswoman for the regulator refused to confirm the number of complaints it had received, but said it would investigate. "We do have rules around impartiality so we will assess any complaints."

Most of the correspondence appears to have been prompted by an internet campaign orchestrated by Liberal Democrat supporters, who used sites such as Facebook and Twitter to encourage people to contact the regulator.

Boulton's intervention also appears to break one of the rules drawn up by broadcasters and party representatives, which places restrictions on the role of the moderator.

Rule 63 states that "It is not the moderator's role to criticise or comment on the leaders' answers". The campaigners believe that Boulton breached rule 63 by "heckling" Clegg.

A spokesman for Sky said Boulton had mentioned the newspaper allegations "as a statement of fact" following a long exchange about MPs' expenses, and denied he had ignored guidelines.

Contacted by the Guardian, Boulton said: "I spoke to Nick Clegg afterwards and I have had no complaints."

The complaints to Ofcom will fuel a growing controversy over the role Rupert Murdoch's media empire is playing in the general election campaign.

Sky News is controlled by Rupert Murdoch's News Corporation, which also owns the Times, the Sun and the News of the World.

The tabloid titles have endorsed David Cameron, and the Sun has been particularly vocal in its support. Last week, the Sun joined other rightwing papers including the Daily Mail and Daily Telegraph and launched attacks on Clegg.

A spokesman for the Liberal Democrats refused to comment.

Clegg's election coordinator Danny Alexander yesterday accused the Conservatives of orchestrating a "negative media smear campaign" in response to the Liberal Democrats' dramatic surge in popularity following the first debate last week.

The list of 76 rules governing the exchanges allow any party to complain to a unit of "senior broadcasting journalists" set up by broadcasters during the debates.

No complaint was made by the Lib Dems during the show.

Complaints from the parties can also be made to various Sky representatives, but the broadcasters said the Lib Dems "did not make one of substance".

The debate, which was hosted by Sky News but was simulcast on BBC News Channel and Sky3, drew a live audience of 4.1 million viewers.

Last Thursday's first leaders' debate on ITV1 attracted more than double the live audience – 9.4 million – and about 40 complaints to Ofcom, mostly from Scottish National party supporters who believe their leader should have been included.

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