Another leaked government document has revealed concerns about potential goods checks between Northern Ireland and Britain after Brexit.
The paper from the Department for Exiting the EU, seen by the BBC, lists preparing for "high levels" of checks as one of the challenges in delivering the government's Brexit deal.
Last week, a leaked Treasury analysis contained a similar warning.
Prime Minister Boris Johnson insisted his Brexit plan was a "great deal".
The prime minister has repeatedly and variously claimed there will be no checks in both directions, but also appeared to change his words on this during the campaign.
The Dexeu memo warns of "security, social and economic impacts" of such checks and controls.
The document was circulated by officials last week, and extracts were first reported by the Financial Times on Monday.
The context of the document appears to be a clear warning from officials that there is insufficient time to get in place the operational systems to implement the renegotiated Northern Ireland elements of the Brexit Withdrawal Agreement by December 2020.
Court challenge warning
It casts considerable doubt on the prime minister's central political argument that Brexit will be "done" in January and fully implemented by December next year.
Another extract seen by the BBC states: "Delivery of the required infrastructure, associated systems, and staffing to implement the requirements of the protocol [Brexit deal] by December 2020 represents a major strategic, political and operational challenge.
"Delivery on the ground would need to commence before we know the outcome of negotiations"
The paper also states that some versions of the Brexit deal in Northern Ireland "could lead to a domestic JR (judicial review)", whereby the government's deal was challenged in court.
There is already an existing legal challenge to be heard in January as to the legality of the Withdrawal Agreement in respect of Northern Ireland's ongoing adherence to all aspects of EU customs law.
The document - titled "Delivering the Protocol by December 2020" - also warns of the "legal and political (domestic and EU) impact of not being able to deliver the protocol in Dec 2020".
The internal warnings shed light on Boris Johnson's current agreement with the EU over the arrangements that will be put in place in Northern Ireland once the UK leaves the bloc.
Under the PM's agreement, Northern Ireland would continue meet almost all EU rules on food and manufactured goods, while the rest of the UK would not.
Northern Ireland would also continue to follow EU customs rules but would, for trade policy, remain part of the UK's customs territory.
"There's no question of there being checks on goods going NI/GB or GB/NI. We're part of the same customs territory and it's very clear that there should be unfettered access between Northern Ireland and the rest of GB," the prime minister told Sky's Sophy Ridge on Sunday.
Mr Johnson's assertion was not supported by a separately leaked internal Treasury report, which detailed new customs procedures required for goods travelling in both directions.
Trade from Great Britain to Northern Ireland would require customs checks as well as safety checks, particularly for food and animal products.
'Strengthen our union'
The prime minister has stressed in particular that goods from Great Britain intended for consumption in Northern Ireland should not be checked or have tariffs, but the mechanisms to distinguish between goods going to Northern Ireland and the Republic of Ireland have yet to be designed, agreed or implemented.
The new Dexeu leak is focused on the difficulty of implementing a deal - which would include building "new facilities" for goods checks in Northern Ireland - by December 2020.
A complicated and unprecedented system of rebated tariffs also needs to be negotiated and delivered by December 2020.
A Conservative spokesman said: "Our new deal with the EU takes the whole of the UK, including Northern Ireland, out of the EU.
"A Conservative majority government will implement this deal and agree a trade agreement next year. In doing so we will strengthen our union and we will not extend the implementation period beyond December 2020."
Arlene Foster, leader of the Democratic Unionist party, said: "If we are to have what has been proposed, then it wouldn't be free flowing trade [within the UK] and that, of course, causes us a great concern."