Britain warned Thursday it would not commit to following European Union rules in a post-Brexit trade deal and threatened to abandon talks in June if no compromise was possible.

Two days after EU ministers approved their own joint approach to the negotiations, which start Monday, Prime Minister Boris Johnson's government set out its own red lines.

Britain has refused an EU push to negotiate a defense and security treaty as part of their new relationship after Brexit, according to sources and documents, highlighting how far apart the two sides are on the eve of new talks. While the EU wants to include external security and defense as one of the parallel strands of talks which would also cover trade, fisheries and student exchanges, among other topics, London refuses to seek a legal agreement on that.

Britain was one of the EU's leading military powers before it left after three years of tortuous divorce talks. It has nuclear arms, a permanent seat at the U.N. Security Council, and access to the Five Eyes spy network with the U.S. These assets make the EU willing to lock Britain into a close and legally binding security partnership.

London has the option to extend the transition beyond the end of 2020 but says it will not. Failure to reach a new agreement with the EU, Britain's largest trading partner, is likely to be hugely disruptive on both sides of the Channel.

In its own negotiating mandate, the EU said Britain must mirror its own standards if it wants to continue freely trading its goods with the bloc's huge single market. But London says this undermines the whole point of Brexit, and officials say they accept an increase in trade barriers as the price to pay for "taking back control."

Johnson, recently elected on a "Get Brexit Done" promise, has called for a looser trade agreement along the lines of the bloc's recent deal with Canada. But Brussels says Britain's proximity and close economic ties forged over five decades of EU membership make it a special case. It fears Britain could undercut EU businesses if it decides to scrap costly commitments to high labor or environmental standards.

Brussels also wants its state aid rules to apply in the U.K. in perpetuity, something London outright rejects.

"We want competition in the future, but it must be fair. Fair and free," EU negotiator Michel Barnier said at an event at the European Parliament on Wednesday. "It should not be a problem for the U.K. to agree on a number of ground rules."

Another flashpoint is on the politically sensitive issue of fishing rights. Brussels wants to maintain the right of its fleets to fish in U.K. waters, warning that failure to agree on this could scupper the wider trade talks. But Britain insists it will be an independent coastal state from January next year and would not link access to U.K. waters to access to EU markets. It is proposing instead that fishing opportunities be negotiated annually, based on stock levels.