THERESA May's deputy has taken a swipe at Boris Johnson as he insisted the Tory leadership frontrunner is not best placed to respond to national emergencies and secure the future of the UK.

Cabinet Office minister David Lidington, the de facto deputy prime minister, said he wanted Jeremy Hunt to be handed the keys to Number 10. 

It came as the race to become the next Prime Minister and Tory leader continued to heat up, with Mr Hunt branding his rival a coward for ducking out of live TV debates.

Mr Lidington said the Union between Scotland and the rest of the UK was "under more pressure" than he had ever known in his lifetime. 

Speaking after addressing a Law Society of Scotland conference to mark 20 years of devolution, he said: "When I try to weigh up in my mind the criteria, it seems to me that first is which man is going to be better to shoulder the security responsibilities that come with the job, the job of Prime Minister?

"The Prime Minister is going to be the person who ultimately decides whether to deploy British military forces. 

"The Prime Minister will be the person who decides how to respond to another 9/11 style attack.

"The Prime Minister is the person who has to decide whether or not to launch a rescue mission to help a British hostage being held, perhaps overseas. 

"The Prime Minister will have to take those decisions often under severe time pressure, and almost inevitably on the basis of intelligence evidence that by its very nature is going to be incomplete. 

"So who has the track record and the temperament that will be best for that?

"Second, I would place the union, and which candidate of the two is going to be best placed not just to defend the union, but to strengthen the union by persuading people who feel perhaps agnostic, or even a bit jaded with the union, that they should give it their full support."

He added: "I will be supporting Jeremy Hunt because I think that on both those issues, but also in terms of other aspects, how I've seen him work, I think he is the better of the two candidates."

Pressed if he was implying that Mr Johnson has the wrong temperament to deal with issues such as national security, Mr Lidington said: "I'm saying, very clearly, from having worked with both of them in Cabinet, they've both held the same very senior role, my judgement is that Jeremy is the one who is best equipped to deal with the 3am call."

The Cabinet Office minister said it was "wrong" and "unwise" for Mr Johnson to duck out of live TV interviews and debates. 

He added: “We’re choosing not just a party leader, we’re choosing a Prime Minister, so i think the country is entitled to know what both the candidates for that office would have as their priorities and how they would go about discharging those responsibilities. 

“So I hope he thinks again and I hope he does agree to take part.”

Asked what he made of Mr Johnson's time as Foreign Secretary, Mr Lidington noticeably paused before singling out his response to the Salisbury poisonings for praise.

Elsewhere, he said a no-deal Brexit would harm support for the Union, and revealed his shock at a recent poll which showed 63% of Tory members would accept Scotland splitting from the UK if it meant securing Brexit.

He said: "To my mind the challenge is not simply from a strong constituency of support for separatism in Scotland or from pressure from a change of demographic in Northern Ireland. 

"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."