MPs are seeing for the first time a series of documents which have been at the centre of a long-running row about parliamentary scrutiny of Brexit.
Studies of how the UK's exit will affect 58 sectors of the economy have been sent to a Commons committee but MPs will not get to see some parts.
The government says market-sensitive information must remain confidential.
Labour has said withholding information is in defiance of the will of MPs and may amount to "contempt of Parliament".
The cross-party Brexit committee of MPs is meeting in Westminster in private session as opposition parties and some Tories call for the documents to be published in full.
What is in the documents?
There has been huge speculation about what is in the Brexit papers, which reportedly run to 800 pages.
No-one has seen them yet, apart from senior ministers and civil servants.
The Commons and Lords Brexit committee are currently getting their first look at them but there are bits which they won't be able to see because they have either been handed over with bits blacked out, or not been handed over by ministers.
The BBC's economics editor Kamal Ahmed says he has been told that the reports are not – as has been claimed by some – in-depth "impact assessments".
He says they will not put a figure on the costs to different industries if there is no comprehensive free trade agreement between the UK and EU after the UK leaves in March 2019.
Instead, he says, the reports will show the size of each of the sectors and their worth to the UK economy and then detail how they work at present within the EU single market and customs union.
Why are they incomplete?
In a letter to the Commons committee, Brexit Secretary David Davis said certain details were being kept private because there was no guarantee all of the 21 MPs on the committee would keep them secret.
"Given we have received no assurances from the committee regarding how any information passed will be used, we have sought not to include commercially, market and negotiation sensitive information," he said.
A "successful outcome" to the talks "requires keeping some information confidential", he argued.
One Conservative member of the committee, Craig McKinlay, has backed this stance, suggesting some of his fellow committee members may "use this information against the national interest".
The cross-party committee, he told the BBC, was "deeply divided" between those "refighting the referendum" and those who want to "move on" and he trusted the government to decide what information to share.
"If we get this wrong, it could cost the country billions," he told BBC Radio 4's Today.
What's all this talk of contempt of Parliament?
Earlier this month, MPs voted for the documents to be released, although Conservative MPs abstained.
Ministers claim the analysis "does not exist in the form Parliament requested" and claim they have satisfied the terms of the parliamentary motion, but Labour disagrees and says the MPs' decision was clearly "binding".
We have always been clear that our analysis does not exist in the form Parliament requested. We have taken time to bring together the analysis we do have in a way that meets Parliament’s specific ask. (2/3)
— Steve Baker MP (@SteveBakerHW) November 28, 2017
End of Twitter post by @SteveBakerHW
Shadow Brexit Secretary Sir Keir Starmer says the government "accepted" the decision at the time and "if they are changing their position now, they are going to have to explain that".
"It was clear that these reports, unredacted [and] in full should be handed over."
While the full picture was not clear yet, he said it "could be a question of contempt of Parliament".
What does that mean? Bringing Parliament into contempt is an ancient term used to describe any act – or failure to act – that may prevent or hinder the work of either House of Parliament.
In theory, any MP could ask the Speaker to rule on the matter and decide whether to refer it to the Committee on Standards and Privileges for investigation. It would ultimately be up to MPs as a whole to decide what action to take, but such a process would take many months.
Laura Kuenssberg's take on the row
Only after the Commons vote did chatter really emerge that the problem was not necessarily that the government was trying to cover up the assessments because they were the stuff of nightmares.
Instead, part of the issue was, in the words of one official, that the work was "embarrassingly thin".
Cue panic, it's suggested in Whitehall, to cobble together in some areas reports that look like serious pieces of work and in other areas to scrape away anything that looks just too grim.
Reading between the lines, it seems that in some departments serious work was done – work that the government has felt necessary to edit judiciously.
But in others, no real heavy lifting was done, so something had to be scrambled.
Finally then, the reports – such as they are – landed with the committee in Parliament, but with a cover letter from the Brexit Secretary telling them that, essentially, the controversial bits have been taken out.
What happens next?
The committee, chaired by Labour's Hilary Benn, is meeting in closed session on Tuesday morning.
They have not been given electronic copies of the documents and the papers, which were kept in a safe overnight in several lever arch folders, will need to be photocopied so it could take some time.
It remains to be seen whether MPs chose to break ranks and go public afterwards with what they've seen.
Among the committee members are a number of vocal opponents of Brexit including Labour MPs Pat McFadden, Emma Reynolds and Seema Malhotra and the SNP's Joanna Cherry. But it is counter-balanced by passionate Brexiteers, including Tories Christopher Chope, Jacob Rees-Mogg and Peter Bone.
The committee's next public meeting is on Wednesday while there could be an urgent question in Parliament before then on the issue.