Pro-Brexit supporters celebrate Brexit outside Stormont in east Belfast, Northern Ireland, Britain on Jan. 31, 2020. Britain officially left the European Union (EU) at 11 p.m. (2300 GMT) Friday, putting an end to its 47-year-long membership of the world's largest trading bloc. (Photo by Paul McErlane/Xinhua)

Britain and Brussels clashed Tuesday over upcoming Brexit talks, with London roundly rejecting key elements of the EU's negotiating platform, setting up an acrimonious start to negotiations next week.

The EU's chief negotiator warned the bloc would not abandon its principles to get a deal and took aim at a British minister as tensions between the two sides ramped up.

Prime Minister Boris Johnson's spokesman fired back, saying Britain would not cave to Brussels' demands for access to fishing waters or its insistence that British companies stick to EU standards on goods.

The talks are to begin in Brussels on Monday in a poisonous atmosphere, with the clock ticking down to a December 31 deadline to reach a deal. Johnson has ruled out seeking any extension.

Britain left the EU in January under the withdrawal agreement struck in 2019, and will be regarded as a non-EU "third country" in the negotiations even though it still trades like an EU member until the end of December.

Without an accord on the new relationship, trade between the EU and Britain would revert to a bare-bones arrangement under WTO rules, causing economic pain on both sides, but especially in the UK.

AFP