LONDON (Reuters) - Brexit trade talks plunged into crisis on Monday after Britain warned the European Union that it could effectively override the divorce deal it signed unless the bloc agrees to a free trade deal by Oct. 15.

In one of the most startling turns of the 4-year Brexit saga, Britain is planning new legislation that will override key parts of the Brexit Withdrawal Agreement - a step that, if implemented, could threaten a treaty signed in January and stoke tension in Northern Ireland.

Sections of the internal market bill, due to be published on Wednesday, are expected to “eliminate the legal force of parts of the withdrawal agreement” in areas including state aid and Northern Ireland customs, the Financial Times said, citing three people familiar with the plans.

Britain has set a deadline of Oct. 15 to strike a free-trade deal with the European Union, and if none is agreed both sides should “accept that and move on,” Prime Minister Boris Johnson will say on Monday.

Johnson will say there is no sense in thinking about timelines beyond Oct. 15.

“If we can’t agree by then, then I do not see that there will be a free trade agreement between us, and we should both accept that and move on,” he will say, according to comments released by his office.

Britain left the EU on Jan. 31 but talks aimed at clinching a new trade deal before the end of a status-quo transition arrangement in December have so far snagged on state aid rules and fishing.

Without a deal nearly $1 trillion in trade between Britain and the EU could be thrown into uncertainty with rules over everything from car parts and medicines to fruit and data.

BREXIT SHOWDOWN

The reported plan to undermine the Withdrawal Agreement - disclosed on the eve of a new round of talks in London - was condemned by parties on both sides of the Irish border.

Irish Foreign Minister Simon Coveney, who played a key role in negotiating the withdrawal agreement and Northern Ireland protocol, said on Twitter that the reported move “would be a very unwise way to proceed.”

Senior members of Northern Ireland’s Sinn Fein and SDLP parties, the region’s two largest Irish nationalist groups, also criticised the British government’s plan, as reported by the newspaper.

If no deal is agreed, Britain would have a trading relationship with the bloc like Australia’s, which would be “a good outcome”, Johnson will say on Monday.

“As a government we are preparing, at our borders and at our ports, to be ready for it,” he will say. “We will have full control over our laws, our rules, and our fishing waters.”

In that case, Britain would be ready to find sensible accommodation with the bloc on practical issues such as flights, lorry transport or scientific cooperation, according to the excerpts.