Prime Minister Theresa May's government published a draft law Thursday to formally end Britain's membership of the European Union, but opposition parties and the leaders of Scotland and Wales threatened to block what they called a "naked power-grab".
The new bill would repeal the 1972 European Communities Act, convert an estimated 12,000 existing European regulations into British law and end the supremacy of EU legislation.
"This bill means that we will be able to exit the European Union with maximum certainty, continuity and control," Brexit Secretary David Davis said.
But ministers are braced for a battle over provisions that give them new powers to amend the EU laws as they are transferred without full parliamentary scrutiny.
These so-called "Henry VIII" powers will be limited for two years, but opposition parties warn they could be used to force through changes to other legislation.
The main opposition Labour Party threatened to hold up the bill with amendments in the House of Commons, while the leaders of Scotland and Wales also warned they could not support it.
First Ministers Nicola Sturgeon and Carwyn Jones said the bill was "a naked power-grab, an attack on the founding principles of devolution and could destabilise our economies".
May's minority government remains fragile one month after the snap June 8 election in which her Conservative Party lost its majority, forcing it to seek an alliance with Northern Ireland's small ultra-conservative Democratic Unionist Party.
In a BBC interview marking one year since taking office after the Brexit vote, May admitted she had shed "a little tear" as the election results came in but said she had never considered resigning.
May began the two-year process of leaving the EU on March 29, setting Britain on an uncharted journey.
Extricating Britain from four decades of membership is no small task, and the new European Union (Withdrawal) Bill is one of eight Brexit bills the government will introduce.
May appealed to rivals to work with her, saying she wanted the "biggest possible consensus".
But Labour's Brexit spokesman Keir Starmer vowed to fight the "sweeping new powers for ministers that are fundamentally undemocratic, unaccountable and unacceptable".
He also warned the bill lacked protections for employment, equality and environmental rights.
Officials do not propose to transfer into British law the EU's Charter of Fundamental Rights, saying everything in the charter will be covered elsewhere.
The small pro-European Liberal Democrats also warned of a battle when the bill is debated in the autumn, with leader Tim Farron saying "this will be hell."
Sturgeon and Jones complained the bill does not provide for the promised transfer of EU powers to their respective devolved governments.
But May's Scottish minister David Mundell said that eventually Brexit would result in a "power bonanza for the Scottish parliament".
The first ministers and Labour leader Jeremy Corbyn held separate meetings in Brussels on Thursday with EU Brexit negotiator Michel Barnier.
The Frenchman insisted he would only negotiate with May's government.
Formal Brexit negotiations began last month and the two sides have already clashed over the future rights of European citizens living in Britain.
The new bill set the stage for another row, as briefing notes suggested its powers could be used to remove those rights if no Brexit deal is reached.
The 3Million, a campaign group representing the estimated number of Europeans living in Britain, said the prospect was "horrendous".
Ahead of the next round of Brexit talks starting next week, Britain also published three new position papers on Thursday, including one confirming that it would withdraw from the European Atomic Energy Community.
May's weakened political position has cast doubt on her Brexit strategy, and Barnier had urged Britain on Wednesday to provide more information, warning that the clock was ticking.
Under the EU's exit procedures, Britain will leave the bloc on March 30, 2019, whether it has negotiated a divorce settlement or not.
The head of Britain's public spending watchdog had earlier criticised the government's lack of unified leadership on Brexit, saying its plans were "vague".
National Audit Office chief Amyas Morse said that if ministers failed to properly prepare for new customs arrangements after Brexit, it would be a "horror show".