Boris Johnson is not as qualified to deal with national security crises as Jeremy Hunt, the de-facto deputy prime minister has warned.

When asked about the two candidates competing to become prime minister, David Lidington said that Mr Hunt was better equipped to deal with terrorist emergencies, hostage situations and the possible deployment of troops.

The Cabinet Secretary, who has served under Theresa May alongside both candidates, criticised the rise of English nationalism and said that the 300-year-old Union of the UK faced a greater risk than he had ever known.

“When I’ve tried to weigh up in my mind the criteria, it seems to me first is which man is going to be better to shoulder the security responsibilities that come with the job of prime minister?” Mr Lidington told the Law Society of Scotland’s conference on 20 years of Scottish devolution.

He said: “The prime minister will have to take those decisions often under severe time pressure and almost inevitably on the basis of intelligence evidence that is, by its very nature, going to be incomplete.

“So, who has the track record and the temperament that will be best for that?”

“My judgement is that Jeremy is the one who is best equipped to deal with the ‘3am call’,” he added.

Leadership favourite Mr Johnson, who has become embroiled in controversy surrounding his personal life after police were called to an apparent domestic row at his girlfriend’s flat, has “got to decide the extent to which he will answer questions about the episode at his home,” Mr Lidington said, urging the former foreign secretary to take part in televised debates with Mr Hunt.

Asked whether Mr Johnson is a “coward” for avoiding TV debates during the leadership campaign, Mr Lidington said: “I think it’s wrong and it’s also unwise for him to duck out of interviews and debates.

“We’re choosing not just a party leader, we’re choosing a prime minister, so I think that 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 agrees to take part.”

British-Irish Council (BIC) – Guernsey
First Minister Nicola Sturgeon with David Lidington (Stefan Rousseau/PA)

The second most important issue facing the candidates is the future of the Union, Mr Lidington argued, suggesting Mr Johnson should assert his unionist credentials “in a way that wins more-general public support”.

Mr Lidington added: “However Boris Johnson is seen in Scotland – and I’ve seen the opinion poll findings – then that actually reinforces the need for him to speak out on the subject.”

“I do think the Union is under more pressure than I’ve known previously in my lifetime. I believe that the value of the Union to every one of the four nations is enormous.

“I believe we are much stronger on the world stage working together as a United Kingdom, and I think that we are significantly stronger economically as one United Kingdom.

“I think that fragmentation of the UK would diminish all four nations. Each of the four nations enriches the whole.

“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.”

Asked about polling showing Conservative Party members would want Brexit delivered, even if it triggers Scotland leaving the UK, Mr Lidington said: “I was pretty shocked when I saw those findings.

“It doesn’t accord with my own experience of talking to party members in my own constituency or in other constituencies around the country that I have visited.

“I sit with Ruth Davidson on this, at the core of being a Conservative is that you believe in the Union of the United Kingdom.

“That to me is something that is more important than almost any other political consideration. So my view is that the splintering of the United Kingdom would be harmful to every part of the United Kingdom.

“Each of us would be diminished economically and in terms of global influence and global opportunity, and therefore 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.”