The Scottish secretary has defended his backing of Theresa May's Brexit deal, saying he was forced to "make a judgement" on the issue.
David Mundell had been criticised for previously stating he would not support a plan that set different arrangements for Northern Ireland after Brexit.
However, the deal includes a so-called "backstop" that could see Northern Ireland tied to EU customs codes.
MPs are due to vote on the plan in the Commons on Tuesday.
Both the SNP and Labour have said they would not support Mrs May's deal, with the SNP calling for the UK to stay in the customs union and Labour saying the UK needed to go back to the negotiating table with the EU.
In October, Mr Mundell joined with Scottish Conservative leader Ruth Davidson to write to the prime minister saying they could not back any deal that introduced separate arrangements for Northern Ireland beyond that which already existed, as it would "undermine the integrity" of the United Kingdom.
However, Speaking on the BBC's Sunday Politics Scotland programme Mr Mundell insisted that the greater threat to the UK came from having no withdrawal deal with the EU before the 29 March deadline.
He acknowledged: "Yes, there will be differences in Northern Ireland, but if the backstop came into place - and if it did - it would be a temporary measure, and I had to make a judgement - do I think that those arrangements coming into place and the temporary nature is a bigger threat to the integrity of the United Kingdom than crashing out of the EU in less than four month's time?
"I'm absolutely clear that the threat to the integrity of the United Kingdom, which I regard as the principal issue, and that is what I set out in the letter to the prime minister.
"The integrity is most likely to be threatened by the crash, by the uncertainty, by the division, by the chaos, which provides the backdrop that nationalists want to move forward their independence referendum here in Scotland."
Mr Mundell, who has served under both David Cameron and Theresa May, also revealed that he would not be in any future UK Cabinet run by Leave supporter Boris Johnson.
He said his disagreements with the outspoken former London mayor would make it "extremely difficult" for him to stay on as Scottish Secretary.
"I think, obviously, given my views about Mr Johnson, which are well known, that would be extremely difficult." He added: "Mr Johnson and I don't agree on a whole range of issues and I don't see myself as being able to serve in that way."
Mr Mundell also said that he could not support a so-called "Norway plus" option, that would see the UK remaining in the European Economic Area and joining a customs union with the EU as it would mean continued membership of the Common Fisheries Policy.
Also speaking on the Sunday Politics Scotland programme, the SNP MP for Glasgow East, David Linden, said the UK government was intent on presenting the House of Commons with a "false choice" over the vote on Mrs May's deal on Tuesday.
He said: "We want to protect jobs in this country. The idea that it is her [Theresa May's] deal or no deal is a false choice.
"We are open to any option that keeps jobs available in Scotland. Remaining in the single market and the customs union - that is out bottom line.
Mr Linden added that Article 50 should be extended in order to get a "coalition of parliamentarians to support staying in the single market and the customs union".
Scottish Labour's Paul Sweeny also said he believed breathing space needed to be found to "properly negotiate a settlement with the EU".
However, he added that it did not "necessarily" mean putting off Article 50.
On what his party would do in parliament next week, Mr Sweeney, who is MP for Glasgow North East, said: "Labour's key priority right now is to bring down this failed deal and end the impasse in parliament.
"Our fundamental point is we need a comprehensive UK-EU customs union to solve the [backstop] problem, and that is where Mr Mundell has failed to address that issue."