British Prime Minister Boris Johnson said Sunday if no agreement is struck by Oct. 15, the United Kingdom will leave without a deal.

The big picture: The U.K. has five weeks to reach a deal with the EU, with negotiations due to resume in London on Tuesday. The threat comes as the U.K. plans legislation to "override" key aspects of the Brexit withdrawal agreement reached with the European Union — including on Northern Ireland, the Financial Times first reported Sunday.

Why it matters: The moves could see the unraveling of the deal U.K. reached with the EU last October to avoid a hard border with customs control on the island of Ireland and the collapse of trade talks, which are due to resume in London on Tuesday.

• Michel Barnier, the EU's chief Brexit negotiator, said last week a "precise implementation of the withdrawal agreement" would be the "only way to avoid a hard border on the island of Ireland and preserve the all-island economy."

Between the lines: Northern Ireland has the U.K.'s only land border with an EU member state.

• Top U.S. Democrats have ruled out a trade deal with the U.K. if Brexit creates a hard border with Ireland and violates the Good Friday Agreement — which helped bring peace to Northern Ireland in the 1990s after decades of sectarian violence. Congress must approve all U.S. trade deals.

Details: Aspects of the U.K. internal market bill, to be published Wednesday, are set to "eliminate the legal force of parts of the withdrawal agreement" signed in January — including on Northern Ireland customs, the FT notes.

• A British government source told the Guardian the move was "part of the preparation for a no-deal exit" that would present "new trade barriers" from Northern Ireland.

What they're saying: A government spokesperson told the FT that officials were working to "resolve outstanding issues" on Northern Ireland. "[W]e are considering fallback options in the event this is not achieved to ensure the communities of Northern Ireland are protected," the spokesperson added.

• Johnson said in a statement, "There is no sense in thinking about timelines that go beyond that point.

Go deeper:Brexit's Irish border headache