David Cameron and Alistair Darling have both cited groups afraid to enter the indyref debate for fear of retribution from the Scottish Government.

The Prime Minister today admits he is "nervous" as campaigning enters its final few weeks, telling the Scottish Daily Mail: "I'm emotional and nervous because it matters so much."

But he also alluded to a "silent majority" of people who were afraid to speak out, citing academics in particular.

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"Recently I was talking to university vice chancellors who are very much part of the silent majority. They don't want to speak out ... because they worry about retribution from the Scottish government," the PM said.

Mr Cameron, who visited a marine engineering firm yesterday, also condemned an attack on Labour MP Jim Murphy, who was egged by a Yes supporter while on a visit to Kirkcaldy.

Mr Murphy suspended his 100-day Scotland-wide referendum tour for Better Together, claiming that Yes Scotland is employing intimidation tactics by co-ordinating mobs of protesters to disrupt the meetings.

Mr Cameron described the incident as undemocratic.

He said: "There's nothing wrong with a bit of heckling but throwing things isn't necessarily part of the democratic process."

Meanwhile, Better Together leader Alistair Darling has spoken about the "inexcusable" scenario whereby people in business, arts and culture are afraid to speak out.

"What's happened increasingly over the last two and a half years, time and time again, I have had people in business - and not just in business, people in the arts and culture - saying, 'I would love to speak out but I don't want to do so because it might affect my business or it might affect my job, my career'.

"This is becoming a huge problem," he told the Daily Telegraph.

He added: "I have had people in business saying 'I don't want to speak out because I have got contracts with the Scottish Government'.

"I've got one guy who was going to give us a donation and he rang up and said, 'Sorry, I have just been rung up by someone in the Scottish Government' - he didn't say who - 'and they said if you ever want a contract from us again you will think twice about it'. His board thought twice about it and he didn't do it.

"It is inexcusable."

Mr Darling said if you are leader of a party of a campaign you need to make it clear there will be "absolutely zero tolerance for this sort of behaviour".