The Scottish Secretary has urged MPs to support the EU Withdrawal Agreement when it is put to a vote in the House of Commons.
More than 80 Tory MPs so far have publicly declared they intend to vote against the plan.
EU leaders signed off on the deal in Brussels on Sunday, following extensive negotiations.
Mr Mundell wrote on Twitter: “There is no credible alternative Deal on offer, only the threat of chaos or more division.
“And that’s what @theSNP want as the backdrop for their #indyref2 agenda.
“So no-deal Brexit wouldn’t just be a disaster for Scotland but also threatens integrity of the UK.
“The Prime Minister made clear earlier today, the UK will be an independent coastal state once again, in full sovereign control of our waters, able to decide for ourselves who we allow to fish in them, with that access not tied to any other aspect of our economic partnership.
“The key point is that we are NOT leaving the EU with a deal on fisheries already in place. There will be a future deal on fishing to be agreed.
“The PM has been clear that she will defend UK fishing interests robustly in that negotiation, just as she has in the Brexit Deal.”
Mr Mundell has been criticised in recent weeks after he threatened to quit the Cabinet if the UK remains tied to the Common Fisheries Policy (CFP) at the end of the Brexit transition period.
Scottish Conservative MP Ross Thomson earlier this month called for Mr Mundell to join other senior Conservatives in quitting the Cabinet over the deal, telling him “no unionist” could support the proposal.
Mr Thomson described the EU’s position on a new fisheries deal with the UK as “deeply troubling”.
Speaking on BBC Sunday Politics Scotland, he said: “That (EU statement) does deeply trouble me.
“The current arrangement is very good for the EU, the arrangement we have on fisheries has been devastating for fishing communities across Scotland and the UK.
“It’s in the interests of the EU to keep it going and they want to build on those existing arrangements so to my mind that can only mean the continuation of some form of the CFP.
“It may not be called that but if it looks like the CFP and behaves like the CFP then it is the CFP and that’s why we need to resist it as we go into these negotiations.”