The DUP has hit back at a former Government special adviser who said the party's decision to vote for Brexit but against the EU trade deal is "a true lesson in how not to do politics".
The comments were made by Raoul Ruparel, former Prime Minister Theresa May's adviser on Europe during the first round of Brexit negotiations.
The DUP has faced fierce criticism from its unionist rivals for playing a role in the creation of a border in the Irish Sea.
As the only main Northern Ireland party that backed Brexit, the DUP opposed Prime Minister Boris Johnson's post-Brexit trade deal because it said the divorce settlement imposes customs checks between Northern Ireland and the rest of the UK.
But in a series of tweets, Mr Ruparel said the DUP have gone "from kingmakers to irrelevant and harming their core cause", adding that this is "a true lesson in how not to do politics."
He tweeted: "As a reminder, they had the offer under the previous government at third meaningful vote of entire UK in a customs union, with alignment on other areas and Stormont lock on new laws.
"As well as a wider economic package. They voted against it and were main reason it failed.
"In the indicative votes process they then proceeded to vote against customs union and European Economic Area. Often professing a desire for the sort of relationship which is now on offer in the Free Trade Agreement.
"They then opened the door to the eventual NI Protocol we have now by backing the PM's original proposals despite knowing the EU would never accept the customs elements and being warned that the priority was flexibility for GB.
"Much of this was based on their entire misreading that, at an election they would still secure their position as kingmakers. But when the election came they did badly and Conservatives outperformed. All this has resulted in barriers to trade between GB and NI, arguably making them more distant than ever.
"All this pains me to say as a unionist but truly a series of epic own goals by the DUP."
The DUP said that as a former Spad to Theresa May, Mr Ruparel's "antipathy to those of us who thwarted their hated Backstop is sadly predictable".
A spokesman added: "His nefarious Backstop was opposed by all unionists across the board. He conveniently forgets that his Backstop created a regulatory border in the Irish Sea and subjected Northern Ireland to the application of the EU Customs Code, all without any consent mechanism as promised by his boss. No unionist worthy of the name could ever agree to his anti Union proposals of the Backstop. Nor did they.
"The problem with his assertion that the rest of the UK would align itself with N Ireland on regulations, and tie itself to a customs union with the EU, is that firstly, unsurprisingly there were never enough of his own party to back it, and secondly, it was not binding on any future Prime Minister (and by that time May had already said she was going)."
They added: "The DUP voted consistently as a matter of principle that the UK should leave the EU together and that no differences should occur unless agreed by both unionists and nationalists in the Assembly.
"Whilst he may disdain our politics we will never abandon our principles."