BRUSSELS (AP) — European Union and British negotiators on Monday entered yet another tension-filled week as they sought a belated post-Brexit trade deal that needs to be vetted and get legislative approval before a Jan. 1 cutoff date.

With both sides refusing to blink on any of the three key outstanding issues, time is increasingly in short supply, especially since the EU approval process would need to take around four weeks.

The U.K. already left the EU on Jan. 31, but a transition period when EU rules apply to trade and other issues runs until the end of next month. Both sides had hoped to get a trade deal by then to save hundreds of thousands of jobs that could be at stake if Brexit amounts to a brutal cliff edge divorce.

As top negotiators resumed meetings in Brussels on Monday, neither side was ready to cede any ground just yet.

"We are working very hard to get a deal, but there is quite a lot to do," said U.K. chief negotiator David Frost as he entered EU headquarters for more talks.

Prime Minister Boris Johnson's office, meanwhile, again set out the strict conditions.

"If we are to make progress in the coming days, we need to see more realism from the EU on what it means for the U.K. to be an independent state," said Johnson's spokesman, James Slack. "There will be no change in our approach."

If there is no change, however, the 27-nation EU insisted it wasn't up to the bloc to do all the compromising. And the EU insisted that it wouldn't be forced into any deal because of time pressure.

"Conditions will have to be right," European Commission spokesman Eric Mamer said, adding that the EU had long-standing red lines where it wouldn't budge.

The bloc accuses Britain of wanting retain access to the EU's lucrative markets, much like any member, without agreeing to follow its rules. The EU fears Britain will slash social and environmental standards, and pump state money into U.K. industries, becoming a low-regulation economic rival on the bloc's doorstep.

Britain says the EU is making unreasonable demands and is failing to treat it as an independent, sovereign state.

At some point, it will come to EU and UK leaders to settle the final divisive issues. And the EU expected that Johnson's self-isolation for two weeks because he came into contact with someone who tested positive for the coronavirus wouldn't affect the talks he has had with European Commission President Ursula von der Leyen.

"There have been regular contacts in any case by phone," Mamer said. "I am sure they will continue to have these contacts."

If there is no deal, businesses on both sides of the English Channel will face tariffs and other barriers to trade starting on Jan. 1. That would hurt economies on both sides, with the impact falling most heavily on the U.K., whose economy is already reeling under the coronavirus pandemic.

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Jill Lawless reported from London.