The European Commission vice president, Maroš Šefčovič, has said British legislation currently before parliament that would empower UK ministers to unilaterally override parts of the Northern Ireland Protocol, is damaging trust and respect with the EU.
In a speech to the British-Irish Association in Oxford, Mr Šefčovič said it was legally and politically inconceivable for the UK government to attempt to modify conditions under which goods can enter the EU single market.
The UK Government is progressing its bill to give ministers powers to scrap the checks on Irish Sea trade created by the post-Brexit protocol.
The Northern Ireland Protocol was jointly agreed by the UK and EU as part of the Brexit Withdrawal Agreement and is designed to avoid a hard border on the island of Ireland.
Mr Šefčovič said the EU had already put forward "far-reaching" proposals to address issues of concern raised by stakeholders in Northern Ireland about the operation of the protocol.
He said the EU proposals would create an express lane for goods moving from Great Britain to Northern Ireland and reduce sanitary and phytosanitary checks and controls by more than 80%.
He said: "These proposals have, however, never been seriously picked up by my counterparts. In short, they were dismissed without consideration. The UK has not even engaged in any meaningful discussions with us since February.
He continued: "Instead, we are now faced with the UK government's continuing determination to push forward the Northern Ireland Protocol Bill, set to disapply core elements of the protocol.
"It is not for the UK government alone to change our bilateral agreement and modify the conditions under which goods can enter the EU's Single Market and reach our consumers", said the vice president.