The Internal Market Bill -- which gives the government power to change aspects of the European Union withdrawal agreement -- has passed its first hurdle in the House of Commons by 340 votes to 263.
The bill will enable free flow of goods and services across England, Scotland, Wales and Northern Ireland after the UK leaves the EU's single market and customs union on 1 January.
Prime Minister Boris Johnson had warned his Conservative party members of parliament who opposed the bill, that Brussels has a “revolver on the table” in the Brexit trade talks.
“Our interlocutors on the other side are holding out the possibility of blockading food and agriculture transports within our own country,” Johnson told members of parliament ahead of the vote. “The EU still have not taken this revolver off the table. I still hope that they will do so.
“The EU still have not taken this revolver off the table. I still hope that they will do so. We cannot have a situation where the very boundaries of our country could be dictated by a foreign power or international organisation.
“No British Prime Minister, no Government, no Parliament could ever accept such an imposition.”
Government ministers say the bill contains vital safeguards to protect Northern Ireland and the rest of the UK, if negotiations on a future trade deal break down.
Several prominent Tory MPs, including former Chancellor Sajid Javid, have said they could not support the final bill unless it is amended. He abstained from Monday's vote.
Johnson's former attorney general Geoffrey Cox has slammed the PM for threatening to damage Britain's "standing and reputation" on the world stage if he pushes through a law overruling key clauses of the original Brexit deal.
The opposition Labor party said the PM was reneging on a deal he himself signed earlier this year, and on which Conservative MPs campaigned in the 2019 election, and was "trashing" the UK's reputation. But a Labor amendment to reject the bill entirely was defeated by 349 votes to 213.
The draft law has brought on an almighty stand-off between the Johnson and EU bosses, who have demanded he withdraw it before they consider negotiating a deal.
The PM has given negotiators until October 15 to come up with an agreement and said the UK will "prosper mightily" with or without a trade deal.
The bill will still have to clear several other stages before becoming law before the transition period runs out and UK leaves the EU at the end of 2020.