The date the UK will leave the EU will be written into law, as Theresa May warns Tory rebels that Brexit will not be derailed.
The UK will leave the EU at 11pm GMT on 29 March 2019, the Government says this will be included in the EU Withdrawal Bill.
Brexit Secretary David Davis said the change will make the UK's departure date "crystal clear".
The Prime Minister issued a blunt message to pro-EU MPs, saying any attempt to block Brexit will not be tolerated.
The move comes as Mr Davis was set to meet the European Union's chief negotiator Michel Barnier in Brussels, where officials have been engaged in the latest round of Brexit talks.
The amendment committing Brexit day in law will be considered by MPs when the bill returns to the Commons next week.
Mrs May's fragile grip on power, relying on DUP votes for a Commons majority, means the Government is vulnerable to any Tory rebellion.
Writing in the Daily Telegraph the Prime Minister said: "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."
Mr Davis indicated that he was talking a "pragmatic" approach to MPs' concerns by writing Brexit day into the Bill.
He 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.
"This important step demonstrates our pragmatic approach to this vital piece of legislation.
"Where MPs can improve the Bill, whatever their party, we will work with them.
Ahead of the meeting in Brussels, Mr Barnier called on the UK to make clear whether it will stick with the "European model" on issues like food and environmental standards and financial regulation when it leaves the bloc.
With a crunch leaders' summit in December, Mr Barnier also said the moment was approaching for a "real clarification" of Britain's position on issues like citizens' rights, the Irish border and the UK's financial settlement.
If the 27 remaining EU members agree next month that sufficient progress has been made on these issues, they will give a green light for negotiations to move on to the questions of trade and transition to a new post-Brexit negotiation.
The Prime Minister will attempt to win support from European businesses for her goal of moving the negotiations on to trade talks. She will meet leading business organisations on Monday to set out her vision of a "bold and deep economic partnership" between the UK and EU after Brexit.