Under this law the UK Parliament passed the European Communities Act. It gave direct effect to EU law and meant that if there was a conflict between an act of the British Parliament and EU law, Parliament lost out and EU law prevailed. https://www.bbc.co.uk/news/business-35766434
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If we repeal this law then we can negotiate a no deal hard brexit on our terms as opposed to their terms.
The UK government announced it was doing this in 2016- given the Brexit deadline of 29th March this must be done sooner (https://www.gov.uk/government/news/government-announces-end-of-european-communities-act)