European Commission vice president Maros Sefcovic said it was "high time we got Brexit done" as he fired the latest salvo against Boris Johnson's government over the Northern Ireland Protocol.
Mr Sefcovic told an audience in London that UK legislation designed to tear up parts of the protocol, which governs Northern Ireland's post-Brexit trading arrangements, simply "would not work".
He also warned that the "shadow being cast" by the row would make Brussels more vigilant about continuing to allow British businesses access to financial services, data and certain food markets.
Ending so-called "mutual recognition" of regulations in those sectors could become a possibility as a result of the UK changing its standards, said Mr Sefcovic, adding that Brussels "will be watching development closely".
The protocol was devised in order to prevent the return of a hard border between Northern Ireland and the Republic of Ireland.
But a consequence is that goods exported to Northern Ireland from Great Britain are subject to customs checks, outraging unionists who are refusing to join power sharing until this changes.
Britain has rejected proposals by the EU to resolve the impasse.
Brussels says these would reduce checks on certain goods by 80%, cut customs paperwork in half and allow the movement of certain goods that would otherwise be restricted such as sausages.
Mr Sefcovic said Brexit had increased red tape and hurt businesses on both sides of the Channel but that the EU now wanted to move on from it "to a truly strategic EU-UK partnership".
He accused Britain of adopting a "my way or the highway" approach.
"We indeed find ourselves in a difficult situation, which will most certainly not simply disappear," he added.
"You may not hear this often from a European Commissioner, but it is high time we got Brexit done."
Mr Sefcovic's comments echoed Boris Johnson's 2019 election pledge.
They came during a speech at the London HQ of financial news and data provider Bloomberg, where in 2013 David Cameron had announced his intention to hold a referendum on EU membership.
Mr Sefcovic said the EU "has its limits" in what it can offer and must protect the integrity of the single market. It was unrealistic and unfair, he said, for the UK to expect "that all barriers can be lifted when goods move to Northern Ireland from the rest of the UK".
"Post-Brexit, the pre-Brexit reality is no longer an option," Mr Sefcovic added.
He said for the UK to "unilaterally change the game and decide what enters the EU's single market" amounted to "a clear violation of international law".
Mr Sefcovic said Britain's plan to override the protocol would "lead to constant uncertainty".
"Ministers in London would have the freedom to change the rules on a whim.
"A dual regulatory regime - where businesses opt for EU or UK regulations - would bury them under a mountain of bureaucracy."
Mr Sefcovic insisted that, with stability and legal certainty about the protocol arrangements, Northern Ireland could be enjoying "jam on both sides of the bread" through its unique position, making it a magnet for investment.
The speech comes days after the prime minister's plan to override the protocol cleared its first Commons hurdle despite fierce criticism led by former prime minister Theresa May.
Earlier this month, Mr Sefcovic responded to the publication of the bill by restarting legal action against the UK for allegedly breaching the EU withdrawal treaty as well as launching two new legal infringement actions.