Downing Street has "wholeheartedly" rejected comments in a memorandum leaked to the press describing cabinet "divisions" over Brexit.
The document, compiled by consultancy firm Deloitte and obtained by the Times newspaper, says Whitehall is working on 500 Brexit-related projects and could need 30,000 extra staff.
But the prime minister's spokeswoman said the work had been "unsolicited".
And Deloitte said there had been no "access" to Number 10 for the report.
No "input from any other government departments" had been received, the company added.
Prime Minister Theresa May wants to invoke Article 50 of the Lisbon Treaty - beginning the formal two-year process for leaving the EU - by the end of March next year.
The government said the leaked memo - entitled "Brexit Update" of 7 November - had been written by a consultant and was not a Cabinet Office document, as reported in earlier versions of this story.
The prime minister's spokeswoman added that someone from the accountancy firm Deloitte had produced it and "the individual is not working for the Cabinet Office on this". The person had never been inside 10 Downing Street and had not engaged with officials since Theresa May had become prime minister, the spokeswoman said.
The document identifies cabinet splits between Foreign Secretary Boris Johnson, Brexit Secretary David Davis and International Trade Secretary Liam Fox on one side, and Chancellor Philip Hammond and Business Secretary Greg Clark on the other.
It says Mrs May is "acquiring a reputation of drawing in decisions and details to settle matters herself" - an approach it describes as being "unlikely to be sustainable".
It says: "Every department has developed a 'bottom-up' plan of what the impact of Brexit could be - and its plan to cope with the 'worst case'.
"Although necessary, this falls considerably short of having a 'government plan for Brexit' because it has no prioritisation and no link to the overall negotiation strategy."
But the PM's spokeswoman said it was "so far removed from the government's assessment". She also "wholeheartedly" disputed the suggestion in the memo that there was no plan for Brexit.
Late on Tuesday afternoon Deloitte issued a statement about the memo saying: "This was a note intended primarily for internal audiences. It was not commissioned by the Cabinet Office, nor any other government department, and represents a view of the task facing Whitehall.
"This work was conducted without access to Number 10 or input from any other government departments."
Labour 'won't block Article 50'
Former Conservative Chancellor Ken Clarke, a prominent supporter of the UK staying in the European Union, told BBC Radio 4's The World at One said of the memo: "I think it's probably entirely accurate. It rings very true."
He added: "It's going to take a good six months to work out how to manage the damage [from Brexit]."
Meanwhile, shadow chancellor John McDonnell said the government's "shambolic" approach to Brexit was failing to equip the UK economy for leaving the EU.
In a speech, he described the chancellor as isolated from cabinet colleagues and "too weak" to make Brexit a success.
However, Mr McDonnell said Labour would not attempt to block or delay the triggering of Article 50 in Parliament.
"To do so would put Labour against the majority will of the British people and on the side of certain corporate elites, who have always had the British people at the back of the queue," he said.
Liberal Democrat EU spokesman Nick Clegg said: "The problem is we don't have any decisions from the government. We don't know what it means when it says, 'Brexit means Brexit.'"
The government is appealing against a High Court ruling that Parliament should have a say before the UK invokes Article 50. The hearing is due to begin at the Supreme Court on 5 December.