The date the UK will leave the EU is to be put into legislation, as Theresa May warns MPs not to try to "block" Brexit.
"Exit day" - 2300 GMT on 29 March 2019 - will be included in the EU Withdrawal Bill, the government has confirmed.
The PM urged parties to "come together" to scrutinise the bill when it returns to the Commons but warned against using it to try to stop or slow it down.
But the author of the Article 50 withdrawal process has said that Brexit could still be reversed.
Cross-bench peer Lord Kerr, who wrote Article 50, the formal procedure for leaving the EU, will give a speech in London on Friday in which he will say: "We can change our minds at any stage of the process".
At an event hosted by the pro-EU Open Britain pressure group, the peer will say: "We are not required to withdraw just because Mrs May sent her letter" to Brussels.
"Actually, the country still has a free choice about whether to proceed. As new facts emerge, people are entitled to take a different view. And there's nothing in Article 50 to stop them."
But Mrs May, in an article in the Telegraph, said the decision to put the date of Brexit "on the front page" of the EU Withdrawal Bill showed the government was determined to see the process through.
"Let no-one doubt our determination or question our resolve, Brexit is happening," she said.
Brexit Secretary David Davis said: "Our amendment makes it crystal clear that the UK is leaving the EU at 11pm on March 29 2019.
"We've listened to members of the public and Parliament and have made this change to remove any confusion or concern about what 'exit day' means."
The bill is designed to transfer EU law into UK law and avoid a legislative "black hole" on the day the UK leaves the EU.
It is facing stiff resistance from some Conservative rebels and from opposition parties, who have described it as a "power grab" by the government.
The PM said the "historic" bill was "fundamental to delivering a smooth and orderly Brexit" and would give "the greatest possible clarity and certainty for all businesses and families across the country".
She said most people wanted politicians to "come together" to negotiate a good Brexit deal and added that MPs "on all sides should come together in scrutinising this Bill".
Mrs May said the government would listen to MPs if they had ideas for improving the bill - as they had done by putting the departure date "on the front page" of the bill.
"But I am just as clear of this: We will not tolerate attempts from any quarter to use the process of amendments to this Bill as a mechanism to try to block the democratic wishes of the British people by attempting to slow down or stop our departure from the European Union."
The draft legislation, formerly known as the Great Repeal Bill, has already passed its second reading, and now faces several attempts to amend it at the next part of its parliamentary journey - the committee stage.
MPs have previously been told there have been 300 amendments and 54 new clauses proposed.