The BBC says UK ministers outlined a bill on Monday aimed at unilaterally changing trade, tax and governance arrangements in the 2019 Brexit treaty.
The treaty was agreed by both sides. The UK now contends that it is disrupting trade and power-sharing in Northern Ireland.
But the EU says overriding parts of the deal would break international law.
European Commission vice-president Maros Sefcovic said there was “no legal or political justification whatsoever for unilaterally changing an international agreement”.
“So let’s call a spade a spade, this is illegal,” Mr Sefcovic said, adding that the UK’s decision “left us with no choice” but to take legal action.
The European Commission said it would restart legal action – paused in March 2021 – over the UK’s decision to delay checks on certain goods arriving into Northern Ireland from Great Britain.
The Commission also launched two new proceedings over claims the UK has failed in its obligations to share trade data and set up border inspection posts.
These legal steps could eventually lead to the UK being fined under a dispute process overseen by the European Court of Justice (ECJ).
In March 2021, the EU launched legal action against the UK for delaying the full implementation of checks on some goods, including agri-food products.
The infringement proceedings were paused last year but have now been resumed by the EU.
If the UK government does not reply within two months, the EU could take the matter to the ECJ, which has powers to impose penalties.
The EU has also launched two new actions for further alleged breaches of the protocol.
These relate to alleged UK failures to carry out its obligations under the EU’s sanitary and phytosanitary (SPS) rules and “provide the EU with certain trade statistics data”.