Brexit Secretary David Davis has denied claims the Government is using EU withdrawal to mask a power grab.
The comments came after the Opposition promised to create "hell" for ministers trying to get the Brexit Repeal Bill through Parliament.
Plans to allow the Government to use so-called "Henry VIII powers", allowing ministers to alter legislation without full parliamentary scrutiny, drew sharp criticism.
But Mr Davis defended the move, telling the BBC: "The bit that is being described as a power grab, the so-called secondary legislation, where things are changed just to make them work, it isn't at the stroke of a pen, it's through secondary legislation, and there are mechanisms whereby the House of Commons, and, indeed, the House of Lords, will have debates on this matter and can vote it down.
"No, it is not just a ministerial signature, it is what they call a statutory instrument which is, can be debated, can be voted on."
Shadow chancellor John McDonnell said Labour would oppose the Bill because it was undemocratic.
He told BBC Newsnight: "It is the big issue. We have taken control from Europe, given it to the Government and then there is no parliamentary discussion or control over that, and that can't be right.
"What we are saying is we need a different type of Bill. I am hoping the Government will let us amend a lot of this Bill and on that basis we may be able to support it. But we can't at the moment because it is so anti-democratic."
With 800 to 1,000 pieces of secondary legislation likely to be brought forward under the powers, and a two-year window in which to exercise them, there are likely to be strong further objections from MPs and peers.
Mr Davis also dismissed talk of a Cabinet clash over how to handle withdrawal from the EU.
Asked if there was disagreement among senior ministers after Foreign Secretary Boris Johnson said there was "no plan for no deal", Mr Davis said he was working on arrangements for all outcomes of Brexit negotiations.
He said: "No, this isn't a clash. I wasn't there when the Foreign Secretary said what he said."
Pressed on why Mr Johnson seemed unaware of the plans, the Brexit Secretary said: "You'll have to ask him."
The remarks came as Labour looked set to vote against the crucial repeal legislation unless it is amended, because it states the European Charter of Fundamental Rights will not be put into UK law after the country's withdrawal from the EU.
The First Ministers of Scotland and Wales, Nicola Sturgeon and Carwyn Jones, said they would not grant the required legislative consent to the Bill as it stands, describing it as a "naked power grab" because it does not immediately return EU powers to devolved administrations.
And the Liberal Democrats warned the Government faces "hell" over the Bill, and a "political nightmare" that could cost Theresa May her job as Prime Minister.
Their statements underlined the fierce battles ahead over the European Union (Withdrawal) Bill, in which a handful of Tory rebels could force the Government to change the legislation when it gets its second reading in the Commons this autumn.
It is understood ministers believe the rights in the charter are already contained in EU rules which the legislation will convert into domestic law on the day of Brexit.
The Bill is designed to transpose EU law into British law so the same rules apply on the day of Brexit as the day before, while giving parliaments and assemblies in Westminster, Edinburgh, Belfast and Cardiff the power to drop or change them in the future.
Mr Davis also indicated there could be some sort of association arrangement with the European civil nuclear regulator Euratom.
The Brexit Secretary signalled that arbitration mechanisms could be put in place to deal with any future trade disputes with the EU rather than the European Court of Justice having the final say.