BRITAIN'S top civil servant has taken a vow of silence when asked by MPs if No 10 had persuaded the Queen to publicly oppose Scottish independence during the 2014 referendum.

Sir Jeremy Heywood, the Cabinet Secretary, declined to comment on reports that No 10 had orchestrated comments by the Queen during the campaign.

The monarch was overheard saying that people should "think very carefully about the future" five days before the vote, in remarks widely interpreted as backing for a No vote, outside Crathie Kirk near Balmoral.

David Cameron was later overheard telling the former New York mayor Michael Bloomberg that after being informed of the 55-45 per cent No vote the Queen had "purred down the line. I've never heard someone so happy."

The Prime Minister was later forced to apologise for revealing details of the conversation with the Queen.

Yesterday, Sir Jeremy told the Commons Public Administration Committee that he would never “politicise” the royal family as he was questioned about the European Union referendum.

Mr Heywood came under pressure from MPs after claims that Prince William had intervened in the debate last month.

The Duke of Cambridge said Britain was an "outward looking nation" with a "proud tradition of seeking out allies" in comments widely seen as backing the EU.

Sir Jeremy was defending the Prime Minister’s decision to ban pro-Brexit ministers from seeing referendum-related documents within their own departments.

Earlier the Northern Ireland Office permanent secretary, Sir Jonathan Stephens, had suggested that NI Secretary Theresa Villiers could access such papers by using a freedom of information request, like a member of the public.

The row has led to fury among Tory ministers.

Work and Pensions Secretary Iain Duncan Smith has reportedly threaded to flout the ban.

But Sir Jeremy defended the decision.

He said: "We don't think it is appropriate - the Prime Minister doesn't think it is appropriate and I don't either - to provide that material to ministers who want to argue against the Government's position so that they can make that case against the Government. That's the material that is being denied to them.”

He added that ministers campaigning to leave the EU would continue to receive a "limousine service" from their officials.

Last month Alex Salmond said that he did not think "for a minute that Her Majesty the Queen actually intervened in the Scottish referendum".

Meanwhile, David Cameron said the EU vote was dividing families as he continued his campaign against Brexit.

But in a comment that could be regarded as a swipe against Boris Johnson, his most high[-profile Tory opponent, he said: "I believe we will be better off if we stay in a reformed European Union.

"And of course, this will be a decision and a choice that divides many people, many communities, sometimes even families, as we debate and think about this."

London Mayor Boris Johnson officially came out for the Leave camp more than a week ago.

His father Stanley Johnson, who worked at the European Commission in the 1970s, is a prominent advocate for the In side.

Chancellor George Osborne confirmed that a comprehensive economic analysis of Britain's EU membership will be published by the Treasury.

Mr Osborne made the announcement as he warned MPs that a vote to leave the EU would result in a "long, costly and messy divorce".