It is now clear that the Attorney General Geoffrey Cox gave the prime minister some stark warnings about what she was signing up to in the EU withdrawal agreement.
His actual advice was much more negative than what was presented to MPs earlier in the week.
Dated 13 November, the letter warns that the UK could remain in a customs union "indefinitely" amid "repeating rounds of negotiations". This is in contrast to the emphasis he placed on the arrangement's likelihood of being "temporary" in the 43-page document published on Monday for MPs to read.
The letter is just six pages long. It is therefore rather baffling as to why the summary of his advice to MPs was 43 pages and included other aspects of the deal such as EU jurisdiction and the budget.
Both Downing Street and DExEU (the Department for Exiting the European Union) say this single letter, which focuses on the Irish backstop, is the sum total of his written advice to Number 10 on the withdrawal bill.
Aspects of his letter have done nothing to quell anger with the government from the DUP.
The attorney general says that under this arrangement, Northern Ireland would be in a full customs union with the EU while the rest of the UK has a slightly different arrangement and this would lead to a "declaration process" at the border between the UK and the EU.
Mr Cox adds: "GB is essentially treated as a third country by NI for goods passing from GB to NI."
DUP leader in the Commons Nigel Dodds describes this as "devastating". It is a step away from Theresa May's promise that there would be no border down the Irish Sea.
In a statement about the withdrawal agreement in the Commons on 26 November - 13 days after receiving her letter from Mr Cox - Mrs May said: "It meets our commitment to ensure there is no hard border between Northern Ireland and Ireland - and also no customs border in the Irish Sea - in the event that the future relationship is not ready by the end of the implementation period."
But the scenario could get even worse from a DUP perspective.
Mr Cox points out in paragraph 26 that under the withdrawal agreement the backstop protocol could be removed "in part" - and that the EU might argue this should be used to remove the UK from the customs union arrangement while keeping Northern Ireland fully in it.
He warns: "GB elements of the customs union fall away, leaving only NI in the EU customs territory."
Once described as "the backstop to the backstop", this scenario has been ruled out by the prime minister, and yet here is her attorney general warning that she has by no means seen off the threat.
Mr Cox goes onto say the arbitration panel of five lawyers set up to decide on such a dispute might not be prepared to make such a "political" judgement.
Despite telling MPs in his summary of the legal advice published Monday that the backstop was designed to be temporary, the tone of the legal advice to the prime minister is very different.
Here the attorney general is unambiguous about the potentially binding nature of the agreement.
He tells Mrs May: "Despite the statements in the protocol that it is not intended to be permanent and the clear intention of the parties that it should be replaced by alternative, permanent arrangements, in international law the protocol would endure indefinitely until a superseding agreement took place, in whole or in part as set out therein.
"Further, the withdrawal agreement cannot provide a legal means of compelling the EU to conclude such an arrangement."
He goes on to warn: "There is a real risk that the United Kingdom might become subject to protracted and repeating rounds of negotiations."
Mr Cox had argued that it was not in the public interest for this letter to be released. That is debatable. What's clear is it was definitely not in the prime minister's interest.