A special advisor to former Prime Minister Theresa May has said he was surprised the Conservative Party "was willing to cut Northern Ireland loose" to exit the EU.
Denzil Davidson was an advisor to foreign secretaries William Hague and Philip Hammon and the UK's European Commissioner Jonathan Hill before working with Mrs May from 2016 until 2019.
He told the think-tank UK In a Changing Europe that "history could have been very different" had former Prime Minister David Cameron not taken the Conservative Party out of a centre-right European Parliament grouping.
Had they remained, "communication would have been much deeper, understanding would have been much broader, there would have been much more willingness to help".
Describing himself as "a patriotic unionist", Mr Davidson said he "had wrongly hoped that genuine unionist commitments were held more widely in the Conservative Party than they now are".
He told the think-tank he was not surprised by Boris Johnson's agreement to an Irish Sea border "because I knew that Northern Ireland was not a priority for him, and that he was willing to cut them loose. I was surprised that the Conservative Party accepted what he agreed to so readily".
Mr Davidson said during the May administration there was "a kind of collective failure in government at the time properly to understand the implications for Northern Ireland, for which I must share the guilt".
"The only guy at the time on our side who really understood the Northern Ireland Brexit problem, and I didn't listen to him enough, was David Lidington."
HE also described a lack of bilateral communication from the Irish government in the early stages as "puzzling", and said negotiators "spent far too little time on the text" of the Northern Irish question and "made mistakes in the whole way that was handled" with the DUP.
But he also claimed the EU "made very little effort at that stage, and I don't think have made much effort since, to earn the unionist community's confidence".