The Scottish government has sought to play down the significance of the letter, and says the firm understood there were risks.
It points to the fact Biffa had taken out a £20m insurance policy against any delay.
Both then first minister Humza Yousaf and Slater accused Lord Jack of “sabotaging” the scheme when it collapsed in 2023.
Lord Jack said the Scottish government should have asked for an exclusion from the Internal Market Act a year earlier when it was clear it was needed.
“I can’t understand why they made such a hash of it,” he said.
“The first minister had written letters, the minister for circular economy too, encouraging stakeholders to make financial commitments when they didn’t have a viable scheme.
“Divergence was very clear from a very early stage. My responsibility was to protect free trade, consumers and business who were very concerned about this in relation to the economy.”
Lord Jack said he was worried about “massive inflation” – more than 1,000 firms had written to him to voice concerns
“I wanted answers to these questions that were going to hit the economy, hit consumers and hit the least well off,” he said.