Prime Minister Boris Johnson told the European Union not to threaten Britain on Saturday, saying a bill which would breach a divorce treaty with the bloc was needed to protect the country's integrity.
With the EU stepping up planning for talks on trade to end without a deal, Johnson has accused its negotiators of threatening to impose a food blockade between mainland Britain and Northern Ireland.
"Let's make the EU take their threats off the table", Johnson said on Twitter. "And let's get this Bill through, back up our negotiators, and protect our country."
British lawmakers will on Monday begin debating the Internal Markets Bill, which one minister has said would breach international law "in a very specific and limited way".
The government says it is needed to clarify the Northern Ireland protocol element of the Brexit deal it signed in January to protect free trade between the four constituent nations of the United Kingdom
But European lawmakers said on Friday they would not approve any new trade deal unless the withdrawal agreement was fully implemented, while there is also talk of possible legal action.
Both sides have set a deadline of the end of October for a deal, raising the prospect that nearly $1 trillion in trade between the EU and Britain could be thrown into confusion at the start of 2021 when a transition period ends.
Writing in the Daily Telegraph, Johnson said a "great deal" could still be done but it appeared the EU were now taking an "extreme interpretation" of the Northern Irish protocol.
"We never seriously believed that the EU would be willing to use a treaty, negotiated in good faith, to blockade one part of the UK, to cut it off, or that they would actually threaten to destroy the economic and territorial integrity of the UK."
Johnson's bill also faces opposition from senior figures in his Conservative Party and some of his own lawmakers who are unhappy at the prospect of infringing international law.
In a video conference call with his lawmakers on Friday he appealed for support for his bill and for them to avoid repeating the "squabbling" over the Brexit divorce deal which saw some quit the party and others thrown out.
Michael Gove, one of Johnson's most senior ministers, said the government had the support of its own lawmakers and those in other parties. But some were clearly unconvinced.
"Unamended I cannot support this Bill ... (it) is damaging brand UK, diminishing our role-model status as defender of global standards," Conservative lawmaker Tobias Ellwood and chairman of parliament's defence committee wrote on Twitter.
Pushed on whether Britain would be breaking international law, Gove said the bill was consistent with "the rule of law" and denied it was a negotiating tactic to put pressure on the EU to make concessions for a trade deal.