A prominent SNP figure has branded the Queen a "political tool" following claims Britain's most senior civil servant and Her Majesty's private secretary intervened to ensure she made a pro-Union appeal to the people of Scotland in the final days of the referendum campaign.

Natalie McGarry, convenor of the SNP in Glasgow and co-founder of Women for Independence, responded to the claims by tweeting: "When the Queen allows herself to become a political tool, she undermines the role she has carved out." She added: "Viva la République."

Ms McGarry, a self-styled republican, is attempting to become the Nationalist candidate in Glasgow East at the May General Election.

It has been claimed Cabinet Secretary Sir Jeremy Heywood and Sir Christopher Geidt for Buckingham Palace were involved in the negotiations that led to the Queen urging voters to "think very carefully" before making their historic decision on September 18.

Senior Whitehall figures were said to have suggested to the palace the Queen should intervene on the matter as polls showed an increase in support for the Yes campaign. However, it was stressed Sir Jeremy and Sir Christopher were aware of the need for any comment by the Queen, who as sovereign is supposed to remain impartial on political matters, not to be too strongly worded but to reflect her support for the Union.

She eventually spoke out, to a well-wisher after the Sunday service at Crathie Kirk, near Balmoral, on September 14, saying: "I hope people will think very carefully about the future."

A Whitehall source reportedly observed: "Without her taking a side, it cast just the right element of doubt over the nature of the decision."

David Cameron was also said to have discussed the referendum with the Queen during a visit to Balmoral a week before her comment. He later caused controversy by suggesting Her Majesty had "purred" down the phoneline after the referendum result rejecting independence was announced. The Prime Minister later had to apologise to her in person for the breach of protocol.

Buckingham Palace and the Cabinet Office declined to comment on the claims about Her Majesty's remarks. Mr Cameron's spokeswoman made clear No 10 did "not comment on conversations between the Prime Minister and the Queen".

Meantime, Republic, the campaign group, called on the UK Parliament to tick off the Queen for making what it called was "a direct intervention in the Scottish referendum".

Stressing it took no view on Scottish independence, Republic nonetheless said it felt the Queen had over-stepped the boundaries of her position as head of state by interfering in the democratic process.

Graham Smith, the group's leader, suggested everyone should be "alarmed at such a political intervention by a hereditary monarch".

He added: "We're calling on MPs to censure the Queen for her intervention in the referendum campaign. Of course, she is damned if she does and damned if she doesn't but the bottom line is that the rules are there and the Queen can't pick and choose when to follow them. This is why our head of state needs to be elected; so if they get it wrong like this they can be held to account."