Development is being “hollowed out” by Boris Johnson, who secretly wants to take back powers from devolved parliaments, John Swinney will claim.
Scotland’s Deputy First Minister will be counted on to “stand up and before it is too late” to defend the transfer to the SNP conference.
Mr Swain is expected to tell members that the Conservative government wants “autocratic Westminster control over Scotland” and is using Brexit to defuse divergence.
But his comments have been dismissed as “just rubbish” by the UK government.
All of us who care so deeply about the Scottish Parliament and its role in Scottish public life need to stand up and count before it is too late
In a virtual speech, he would argue that “the only way to retain the advantage of deviance is to be independent” and suggest that decisions made in Holyrood may be “regularly underestimated by the Tories”.
Mr Swain is expected to say: “We are now seeing a concerted attack on our parliament, which was fought tooth and nail against what was established in the first place.
“In a bizarre irony, the Tories are using something the Scottish people voted overwhelmingly against – Brexit – to undermine something they voted overwhelmingly for the Scottish Parliament.
“Through the Internal Market Act, the Tories have given themselves unfettered power to decide the rules of the internal market – and completely ignore transfer agreements if they so desire.”
Claiming that the prime minister may wish to reverse the divergence, Mr Swain would add: “There will be no sudden moments of the Big Bang, Boris Johnson is not going to stand up and announce to the enthusiastic Tories that he will Terminating the transfer agreement – much like I’m sure he would like.
“But make no mistake – piece by piece, the transfer is being filmed silently, hollowed out inside by a Tory party that has always resisted the idea of anything other than Westminster control over Scotland.
“Like the climate emergency, which is slowly creeping up on us every day – we need to issue a code red for transfer.
“All of us who care so deeply about the Scottish Parliament and its role in Scottish public life need to stand up and be counted before it is too late.”
A UK government spokeswoman said: “This is just nonsense.
“The Internal Markets Bill ensures that vital trade between the four corners of the UK can continue uninterrupted.
“Now more than ever, people in Scotland want to see the UK and Scottish governments working together to protect lives and livelihoods.
“The pandemic, and our collective response – from furlough planning to vaccine procurement and support for our military personnel – has shown that we are strongest when we work together.”