Scottish independence and a push for Irish reunification are "real threats" of a no-deal Brexit, International Trade Secretary Liam Fox has said.
The Brexiteer is backing Foreign Secretary Jeremy Hunt in the race to become prime minister.
Both Mr Hunt and rival Boris Johnson have threatened to take the UK out of the EU without a deal if a fresh one cannot be brokered.
Dr Fox was challenged on BBC Radio 4's Today programme on whether a badly handled EU exit without a deal could lead to Scottish independence and a border poll to reunite the island of Ireland.
He said: "We know they are both real threats, there's no point in pretending they're not there.
"And therefore we have to put in place the details of what a no-deal would look like, including how we protect crucial industries like fishing and farming and how we deal with small businesses.
"It's not good enough to simply say we are going to have a no-deal; we need to prepare about it, that's what government is all about."
Former foreign secretary Mr Johnson has pledged Britain will leave the EU by October 31 "do or die", but said he believes the chances of a no-deal Brexit are a "million-to-one against".
Mr Hunt has argued it is a "fake deadline" and that if a deal is close he would be prepared to ask for a delay.
Both are firmly against any break-up of the UK, while Mr Johnson has argued that a well-performed Brexit could "cement and intensify" the union.