The UK has said it is hopeful of securing a comprehensive deal modelled on the EU's arrangement with Canada, but Sir John said it was "disingenuous" of ministers to pretend they were not seeking far deeper commitments in key areas, such as energy and aviation.
He said he feared, as a result, the process would end up either with no deal or a "flimsy and bare-bones" agreement that created new barriers to trade and would be a "wretched betrayal" of the promises made to British voters during the 2016 referendum.
"These costs and complexities are the certain legacy of Brexit," he said. "This is as a result of our negotiating failure - and it is a failure.
"Because of our bombast, our blustering, our threats and our inflexibility - our trade will be less profitable, our Treasury poorer, our jobs fewer, and our future less prosperous."
He added: "It now seems that on 1 January next year, Brexit may be even more brutal than anyone expected."
Sir John, from BBC News.
10s of 1000s of those who voted for Brexit were lied to, betrayed in a massive way. There are some, a small proportion, some right here on GN- who would have voted for it whatever the consequences, whatever the cost, whatever the damage. But I estimate it to be currently a very small minority. Even Rees Mogg admitted it would never ever pass today, now people know, that all the promises listed above by Sir John, were either pure lies, or Johnson and co have changed things since. They would have never voted for a Withdrawal Agreement signed with fingers crossed behind the back and every intention to break International Law. Johnson just pretended to sign the WA on agreed basic terms for the negotiations- and has now totally scuppered our chances of a fair deal, and ruined the reputation of the UK for generations to come. Few would vote for that NOW- knowing what they do.
Yes, I know there is a handful here on GN who would. It surely says a lot about their principles and ... other stuff.