Taking back powers from the European Union is set to tie up Westminster for up to a decade, it has been warned.
Weeding out unwanted legislation and putting in place powers to replace those that will disappear after Brexit will require "considerable" resources, according to a parliamentary expert.
The overhaul will put pressure on the civil service, which has faced significant cut backs under Prime Minister David Cameron.
It will also mean the need for new bodies, such as an agriculture payments system, to be created just years after the Government pledged a bonfire of the quangos.
Hannah White, a programme director at the Institute for Government, estimated it could take 10 years to sort out legislation.
She said: "Brexit poses a challenge for Whitehall and Westminster.
"Extricating the UK from its membership of the EU, establishing a new relationship and taking back responsibility for a range of significant policy areas previously dealt with by Brussels - such as agriculture and fisheries - will require considerable staff resource, expertise and time.
"This will be challenging in the context of a shrinking civil service. Leadership will be key, as will co-ordination, including with the devolved administrations.
"Depending on the UK Government's decisions about what EU policies and regulations it wants to retain, amend or discard, a lot of Parliament's time will be taken up for years dealing with the legislative changes required."
The European Communities Act (ECA) 1972 is the main piece of UK legislation that ensures the supremacy of Brussels laws.
EU directives are passed into the system through secondary legislation. Regulations, however, take effect without requiring legislation in Westminster, meaning rules in areas like environmental standards will disappear if the ECA is repealed.
Some Leave supporters have suggested bypassing the need to invoke Article 50, which sets the terms of the divorce between the UK and the EU, by swiftly ditching the ECA to sever Britain's ties with Brussels.
But such a provocative move, which would go against the UK's obligations under the Lisbon Treaty, risks leaving the country's reputation for sticking to its agreements seriously damaged at a time when it wants to negotiate new international trade deals.
Repealing the ECA once Article 50 negotiations have been carried out opens up swathes of policy areas for review, including agriculture, fisheries and financial services.
The House of Commons Library said there was "no totally accurate, rational or useful way" of calculating how many national laws are based on or influenced by the EU.
It is "possible to justify any measure between 15% and 55%", it states.