Government departments spent more than £4bn on preparations for leaving the EU, says the public spending watchdog.
The National Audit Office said this figure included spending on staff, external advice and advertising.
A Treasury spokesperson said the government had made "all necessary funds available" to ensure the country was prepared for leaving the EU.
But the Lib Dems claimed "billions of pounds have been thrown away in a bid to paper over the Tories' Brexit mess".
The NAO stressed in its report that it was not making a judgement on whether the spending represented value for money.
It also emphasised that the figures represented a "minimum estimated level of spend" due to "limitations" in the data provided by departments.
The spending watchdog's report found that the Home Office, HM Revenue and Customs and the environment department accounted for more than half of the £4.4bn spent on Brexit preparations.
Between 2016-17 and 2019-20, the Treasury made available £6.3bn of additional funding to cover the costs of the UK leaving the EU with or without a deal.
Of this money, at least £1.9bn was spent on staffing. The NAO said staffing levels peaked in October 2019 when 22,000 civil servants were working on Brexit planning.
Advertising, building new systems and other services cost at least £1.5bn - this included spending £283m to build the EU settlement scheme and £69m on Operation Brock, a traffic management system to be used in Kent in the event of a no-deal Brexit.
The £1.5bn also covered the £46m spent on the government's "Get ready for Brexit" campaign. Earlier this year the NAO said it was "not clear that the campaign led to the public being significantly better prepared".
Expertise and external advice cost £288m while local government organisations received £104m.
Government departments have also reported £92m in losses relating to Brexit - this includes £50m paid to ferry companies and £33m to Eurotunnel.
'Lack of transparency'
The chairwoman of the Commons Public Accounts Committee, Meg Hillier, said: "The public has been kept in the dark as to what the Government has been doing.
"Data is limited, and the Treasury seem unconcerned by the lack of transparency."
Liberal Democrat Brexit spokesman Alistair Carmichael said: "In the face of major floods and the coronavirus threat, we have to ask if the government knows its own spending priorities."
The head of the NAO, Gareth Davies, said: "In preparing for EU exit, government departments planned for multiple potential outcomes, with shifting timetables and uncertainty.
"Producing this report has highlighted limitations in how government monitored spending on EU exit specifically, and cross-government programmes more generally."