Britain needs to restore trust by fully implementing the Protocol governing trade for Northern Ireland, European Commission vice-president Maroš Šefcovic has said.
He warned: "Our patience cannot last forever. The clock is well and truly ticking."
Britain should show "unwavering commitment" to implementing the Northern Ireland Protocol, rather than "continually putting it into doubt", Mr Šefcovic said in a speech at the College of Europe.
He also called for jointly-held investment conferences to "install confidence in the business community in Northern Ireland and pave the way to further opportunity".
Mr Šefcovic added: "We cannot do this alone. It has to be a joint endeavour between the EU and the UK."
Mr Šefcovic said Britain should not make any further unilateral changes to the trading arrangements, as it did in March, and instead favour joint action through joint bodies.
He also welcomed a British bid to co-operate over sausages.
"I welcome that the UK is recognising the value of this approach on one of the outstanding issues, the supply of chilled meats from Britain to Northern Ireland because what the Protocol truly embodies is trust," he stated.
If unilateral action is taken "I fear a downward spiral in our relations ... and in this case the EU will not be shy in reacting firmly and resolutely to ensure that the letter and the spirit of the Protocol are respected."
The EC vice-president said the EU had "an unshakeable commitment to the people of Northern Ireland to ensure that the peace, stability and prosperity they have enjoyed over the last 20 years is preserved".
The UK has asked the EU for a further three months to resolve a dispute over whether chilled meat products, such as sausages, that are made in Britain can be sold in Northern Ireland.
The EU said it would consider the request.
The European Union does not allow imports of chilled minced meat and "meat preparations", which include sausages, from third countries, which Britain became after leaving the bloc.
Northern Ireland remains in the EU single market for goods, an arrangement designed to keep the border open and maintain peace.
As such, sausages will be barred from Britain when a grace period expires at the end of June.
"Everyone around the table understood what this compromise meant in practice," Mr Šefcovic said.
"The EU will not and cannot accept this delicate balance being unilaterally changed or disapplied because of buyer's remorse."
Mr Šefcovic said the EU was willing to find creative solutions.
"We turn our rules upside-down and inside-out to find solid solutions to outstanding problems, but these solutions will only work if they are taken up," he added.
Minister for Foreign Affairs Simon Coveney said on Twitter that trade and investment opportunities for the North from the protocol are "significant".
Trade & investment opportunities for NI from Protocol r significant, with full access to UK&EU markets.— Simon Coveney (@simoncoveney) June 18, 2021
For sure, there r problems with protocol implementation, but with UK/EU partnership & pragmatism they can be overcome.
- There's far more opportunity than risk. https://t.co/lFU2oT76hP