MPs must "work together" on Brexit, the minister in charge of the UK's EU exit has said, as he publishes a bill to convert EU law into British law.
The legislation, known as the Repeal Bill, will ensure the same rules apply in the UK after Brexit, while giving UK parliaments the power to change them.
Brexit Secretary David Davis said he will "work with anyone" to make it a success, but he faces opposition.
Liberal Democrat leader Tim Farron told the government: "This will be hell."
Labour vowed to vote against the legislation unless there were significant changes to the details previously set out, while the SNP said there needed to be "clarity" over which powers repatriated from the EU should go to the devolved nations.
The Conservatives are relying on Democratic Unionist Party support to win key votes after losing their Commons majority in the general election.
BBC political editor Laura Kuenssberg said there could be "parliamentary guerrilla warfare" on the bill.
She told BBC Radio 4's Today: "For opposition parties and for Remainer Tories there is a sense today of 'here we go'. This is government critics' first big chance, bit by bit in Parliament, to try to put their version of Brexit, not Theresa May's, on to the statute book".
Formally known as the European Union (Withdrawal) Bill, the draft legislation is a key plank of the government's Brexit strategy.
It will repeal the European Communities Act 1972, which took Britain into the EU and remove the supremacy of Brussels law.
The bill will also temporary powers to correct laws that will not operate appropriately after Brexit.
It is not expected to be debated until the Autumn, but will need to have been passed by the time the UK leaves the EU - due to be in March 2019.
Mr Davis said it would allow the UK to leave the EU with "maximum certainty, continuity and control".
"It is one of the most significant pieces of legislation that has ever passed through Parliament and is a major milestone in the process of our withdrawal from the European Union," he said.
"By working together, in the national interest, we can ensure we have a fully functioning legal system on the day we leave the European Union.
"The eyes of the country are on us and I will work with anyone to achieve this goal and shape a new future for our country."
'Pushed, prodded, cajoled'
However, Labour says there is not enough accountability over the "sweeping delegated powers" ministers will give themselves to alter legislation and claims the Bill represents a "power grab" by Westminster over the devolved administrations.
Labour MP Hilary Benn, who is chairman of the Brexit select committee, said there are "real and serious concerns about the way in which this is done".
He told BBC Radio 4's Today programme: "The government needs to understand that Parliament is going to be an active participant in this process.
"I have to say, at every stage thus far ministers have had to be pushed and prodded and cajoled into taking Parliament's role seriously."
an Blackford MP, the SNP's Leader at Westminster, said: "The UK government needs to provide clarity over the repatriation of powers currently with the EU and which should go to the devolved nations.
"People in Scotland deserve a commitment from the UK Government to categorically rule out the threat of a Westminster power grab of devolved powers at any time as a result of the Repeal Bill and the UK Government should give a cast iron guarantee that devolved powers will be increased."
Mr Farron said he was "putting the government on warning", promising a tougher test than than it faced when passing legislation authorising the UK's departure from the EU.
"If you found the Article 50 Bill difficult, you should be under no illusion, this will be hell," he said.
"If the government try any wheeze or trick to force through changes to vital protections, from workers' rights to the environment, they are playing with fire."
