There was a 'Plan B', it's been revealed, to ensure the smooth running of the country if Boris Johnson had died of coronavirus or became too ill to return to Downing Street.
Sir Mark Sedwill, the outgoing Cabinet Secretary and National Security Advisor, has said that consideration was given to the worst-case scenario and the prime minister approved the plan himself.
"Continuity of government of course is one of the key principles of anyone's constitution, and so we did make sure we had in place the measures necessary," Sir Mark has said.
"Given he was going into intensive care and needed treatment while he was there, that's why we... we talked to him about this in advance so it was on his authority, that Dominic Raab came in as First Secretary of State and essentially assumed most of the responsibilities of the prime minister.
"It wasn't just, God forbid, if his condition had deteriorated, but while in intensive care it wasn't possible for him to exercise all of the functions of the premiership, particularly of course if there'd been a crisis."
Sir Mark was speaking on the podcast series Off the Record with Alistair Bunkall.
It is his first and only interview in office, and highly rare for serving cabinet secretaries to speak so openly.
Asked about the night Britain went into lockdown, he said the mood in Downing Street was "sober" and there was "a real sense that this was a really significant national moment".
Challenged on whether lockdown was imposed at the right moment, Sir Mark admitted that would be the key question for a future inquiry.
"What I can be clear about is that all of those issues were very actively discussed. Were we heading into lockdown fast enough? Were we going too fast? Ministers were always having to balance off the potential economic consequences, the compliance among the public and so on.
"These decisions look in hindsight sometimes more obvious than they do at the time. At the time you're dealing with incomplete information, against a virus the full nature of which was still not entirely clear, with all the best advice that you can.
"There were very active decisions in that period. I think only time will tell whether that came at exactly the right time or not. What I can say is everyone thought very hard about it going into that decision."
Sir Mark is leaving his role this week after just over two years in the job.
He was appointed by Theresa May after the death of the last cabinet secretary Sir Jeremy Heywood.
Although he has spent significantly less time in office than many of his predecessors, Sir Mark rejected suggestions he was forced out early.
"There's a lot of speculation around that, and I've spoken about how challenging it is for civil servants particularly, since we don't traditionally answer back, to have some of that political speculation around about you, the sniping in the press, and so on.
"But I have good relationship with the PM. He and I agree this. I actually took the initiative on the timing itself. And as you've seen, it's all very amicable.
"It always made sense that when we decided that the cabinet secretary role and the national necurity role should be split back out again that that would be the time for me to move on. It will always be determined by events, so probably early than I expected, but definitely not earlier than I'd hoped."
Sir Mark isn't the only senior civil servant to leave office in recent months - six Whitehall permanent secretaries are departing their posts, giving rise to the suggestion Boris Johnson and his senior adviser Dominic Cummings are politicising the civil service. He denied this.
"I think it is natural that there is that kind of churn, but I don't want to get into every individual decision that's been that's been made either by the individuals themselves or about them.
"This government, like others, wants to ensure there is proper political leadership of government. That doesn't mean politicising the civil service. No one's ever asked me, for example, which way I voted. I've never seen a desire to have a civil servant of a particular political affiliation."
But Sir Mark did say that the independence and impartiality of the Civil Service had come under pressure during the Scottish and EU Referendums:
"It certainly has been challenged, and I think the 2014 and 2016 referendums probably meant that it was challenged, in that those were essentially so existential a set of questions about the United Kingdom's national identity and place in the world...
"That particularly the protagonists on either side of that found it hard to retain the confidence that everyone - whether it's the civil service or indeed in other parts of government - was simply discharging their responsibilities with the neutrality and impartiality that we prize.
"There was a presumption that people were taking one side or the other and I've been criticised by if you look at social media, I've been criticised for being a for being presumed to be on one side of the other of those of those arguments and others have others have too.
"So I think it has been challenged has been challenged because of the issues that have dominated our politics for the past decade."