DAVID Cameron has indiscreetly let slip how pleased the Queen was at the Scottish referendum result, which kept her kingdom united.

In an unintended breach of convention, the Prime Minister was caught on camera claiming Her Majesty had "purred" down the line after he informed her that Scotland had voted against independence.

In New York, he was heard telling Michael Bloomberg, the city's former mayor, of the relief he felt at not having to inform the Queen that Scotland had left the United Kingdom.

He smiled broadly as he recalled how he was able to tell her it was "all right" after the referendum resulted in a No victory.

"The definition of relief, if you are Prime Minister of the United Kingdom, is ringing up Her Majesty the Queen and saying: 'Your Majesty, it is all right, it's okay. That was something. She purred down the line."

Mr Cameron went on: "But it should never have been that close; it wasn't in the end. But there was a time in the middle of the campaign when it felt..."

Mr Bloomberg spoke before the Tory leader finished his sentence in a period of the recording which is inaudible as the two men walked through the offices of Mr Bloomberg's media empire.

The PM added: "I've said I want to find these polling companies and I want to sue them for my stomach ulcers because of what they put me through. It was very nervous moments."

A senior Labour source said it had been "disrespectful" of Mr Cameron to have let the cat out of the bag.

Meantime, a spokesman at Buckingham Palace said: "We would never comment on a private conversation between Her Majesty and the Prime Minister."

Just four days before the referendum, the Queen was reported as saying that he hoped Scots would "think very carefully" about the vote. She made the comment as she greeted well-wishers outside Crathie Kirk, near Balmoral Castle.

On Friday, after the outcome of the vote was announced, the monarch issued a statement saying that Scotland's vote to stay in the Union was "a result that all of us throughout the United Kingdom will respect".

She added: "As we move forward, we should remember that despite the range of views that have been expressed, we have in common an enduring love of Scotland, which is one of the things that helps to unite us all.

"Knowing the people of Scotland as I do, I have no doubt that Scots, like others throughout the United Kingdom, are able to express strongly held opinions before coming together again in a spirit of mutual respect and support, to work constructively for the future of Scotland and indeed all parts of this country.

"My family and I will do all we can to help and support you in this important task."