The BBC's flagship current affairs programme has been accused of anti-unionist bias by the chairman of a Westminster committee scrutinising the SNP`s plans for independence.

Labour MP Ian Davidson, who leads the Scottish Affairs Committee, referred to BBC Newsnight Scotland as "News-Nat" throughout an interview with the programme last night.

Host Isabel Fraser denied the accusation and called for an apology, to which Mr Davidson replied: "Certainly not."

Mr Davidson was defending the committee`s position that the Scottish Parliament cannot presently legislate to hold a referendum on "separation" and that agreement should be reached between Holyrood and Westminster to create the necessary legal powers.

The Scottish Government insists it would welcome the legal powers - known as a section 30 order - provided it comes with "no Westminster strings attached".

Mr Davidson said: "I understand that News-Nat Scotland's position is that the powers should be given to the Scottish Parliament, and that the SNP should do as they wish."

Ms Fraser said it was "ludicrous" to suggest that she had predicated her question on a "preconception".

Mr Davidson replied: "I don't believe it is. There is a general political view that News-Nat Scotland is biased in favour of the Scottish Parliament handling all these powers themselves."

He said the committee should have responsibility for reviewing, supervising and assessing the proposed section 30 order, and would "of course" attach conditions to it.

He argued that Westminster has the "moral authority" to ensure the referendum result does not end up in the courts.

Ms Fraser said: "Perhaps it might be appropriate at this stage if you would like to apologise to me for suggesting that I have come into the interview in any way biased against your argument or for one political party or another.

"Nobody in this programme works in that way and it is offensive that you should suggest that."

Mr Davidson responded: "Certainly not. I don't believe that.

"I have already complained, as has the Labour Party, on a number of occasions about the way in which News-Nat Scotland behaves.

"I think you are clearly biased and have been for a long time against the unionist parties, and if that causes you concern then I'm afraid that you've just got to recognise that politics is about people exchanging views.

"You are not above the fray. If you want to stand for election, do so. Otherwise, try to be more neutral."

Ms Fraser said: "I absolutely reject what you say and I think there is plenty of evidence to refute that."