David Cameron said he asked the queen to show her opposition to the independence

Former British prime minister David Cameron revealed Thursday that he asked Queen Elizabeth II to intervene in the referendum on Scottish independence -- despite her neutral role as head of state.

Cameron, who led the successful campaign for Scotland to stay in the United Kingdom in the 2014 referendum, said he asked if she could offer even a "raising of the eyebrow" to indicate her opposition to independence.

He told the BBC he spoke with the queen's private secretary, "not asking for anything that would be in any way improper or unconstitutional, but just a raising of the eyebrow, even, you know, a quarter of an inch -- we thought would make a difference."

A few days before the vote, the queen told a member of the public in Scotland that she hoped "people would think very carefully about the future" -- comments that made headlines.

Scotland voted by 55 percent to stay in the UK.

In a BBC documentary to mark the publication of his memoirs, Cameron said that "although the words were very limited, I think it helped to put a slightly different perception on things".

It is not the first time that the former Conservative leader has been indiscreet about his dealings with the queen, which by convention should remain secret.

He was caught on camera describing how the monarch "purred down the line" after he phoned to inform her that Scotland had rejected independence.

Former Scottish National Party leader Alex Salmond said the request for an intervention "is not only totally improper but an indication of how desperate prime minister Cameron" was in the referendum campaign.

Despite losing the 2014 vote, the independence movement in Scotland remains active, particularly since the 2016 Brexit referendum.

Scots voted to stay in the European Union but the UK as a whole voted to leave.

Cameron had led the anti-Brexit campaign and stepped down when he lost.