The Scottish National Party has said it will vote against and brexit deal that takes the UK out of the single market and customs union.
Ian Blackford has been reported as saying the SNP's 35 MPs would also resist a no deal brexit or a blind brexit where nothing is known about the deal.
Blackford addressed delegates at the SNP's annual conference saying "The only deal we can accept is one that keeps Scotland in the single market or customs union. Any other deal sells us short. I put the prime minister on notice that we will not support any measure that threatens Scottish jobs or living standards."
Nicola Sturgeon has also warned it is "almost inevitable" that the SNP would vote against whatever deal May produced.
Blackford has said he believes Brexit will cause lasting harm and threaten living standards across the UK.