Theresa May has been warned not to use her planned Great Repeal Bill to avoid full parliamentary scrutiny of post-Brexit changes.
The legislation is aimed at transposing EU regulations into domestic law which can then be amended after the break from Brussels.
But the Prime Minister was warned against using so-called Henry VIII powers in the Brexit legislation - particularly after one of the central themes of the Leave campaign was for the UK Parliament to "take back control".
The powers, named after the Tudor monarch, enable Acts of Parliament to be amended or repealed by delegated legislation without following the process for introducing new laws.
A report by the Hansard Society said delegated legislation will be "central to the delivery of the objectives of the Great Repeal Bill" but scrutiny of such measures was "not fit for purpose", particularly in the Commons.
Hansard Society director Ruth Fox said: "After the EU referendum claims about 'taking back control' and 'reclaiming parliamentary sovereignty' are being widely bandied about.
"But enabling ministers to amend or repeal primary legislation by delegated legislation with little or no scrutiny undermines the principle of parliamentary sovereignty and a fundamental argument of Leave campaigners - namely that Parliament is the supreme, sole legislative authority with the power to create, amend or repeal any law.
"If the Government is going to 'repeal, amend and improve' EU-related law through the Great Repeal Bill, Parliament needs to have procedures in place to scrutinise the proposals effectively. If not, there is a very real threat that the Brexit process will empower the Government rather than Parliament."
She added: "Brexit poses the biggest legislative challenge Parliament has ever faced; if it is to fulfil its responsibility to hold the Government to account MPs need better procedures and the commitment to step up and do a better job.'
Senior Liberal Democrat Tom Brake said: "Brexit will be the most important issue facing our country for years to come, and attempts by Theresa May to prevent full parliamentary scrutiny are anti-democratic and wrong.
"The Great Repeal Bill will impact on every sector of society and the economy. It is vital that Parliament has the opportunity to discuss all these impacts properly. Simply trying to slip through major changes in policy, via secondary legislation, is unacceptable and should be rejected by Parliament.
"The Liberal Democrats are fighting for a Britain which is open, tolerant and united. We will not hesitate to reject changes to legislation which might compromise that vision."
A Government spokesman said: "The Great Repeal Bill will end the authority of EU law and return power to the UK.
"Crucially, it will also enable the Government to ensure that the statute book functions effectively on the day we leave. That will require both subsequent primary and secondary legislation.
"Parliament will have every opportunity to debate and scrutinise the Bill during its passage."