The de facto deputy prime minister said he was shocked by a poll from YouGov which found that 63 per cent of UK Conservative Party members would prefer to see Scotland leave the UK than Brexit abandoned.

He said a no-deal Brexit would bring the Union to the brink.

The Cabinet Office minister, who is backing Jeremy Hunt’s bid for Number 10 against Boris Johnson, said preserving the Union “was more important than almost any other political consideration”.

Mr Lidington, 62, said: “I feel it is the duty of the next prime minister to do all in his power to uphold and strengthen the integrity of the United Kingdom and to win public support for that.

“It is in part from an English mood that is sometimes indifferent to the Union and unaware that it is the Union, the efforts of the United Kingdom as a whole, that can achieve far more than England on its own.”

He later said: “I am a very strong unionist, but I think we need to be alive to the fact that there is a combination of nationalist feeling on the one hand, and indifference towards or ignorance of the value of the Union on the other, that puts that achievement at risk.”