Britain's reputation is in danger of being damaged if it breaks its word on Brexit, the EU's former chief negotiator has warned.
Speaking to French radio on Monday Michel Barnier urged Boris Johnson to "respect his signature".
It comes after the the UK unilaterally overrode parts of the withdrawal agreement negotiated with Mr Barnier's team to better suit British businesses.
Unionists and companies in Northern Ireland say the Northern Ireland protocol and the new checks it introduces that make trade with Great Britain more difficult is damaging to their interests.
Mr Barnier, who retired from the European Commission earlier this year, told France Info radio: “The United Kingdom needs to pay attention to its reputation.
“I want Mr Johnson to respect his signature."
Lord Frost, the UK's Brexit negotiator, last week said the protocol had been more damaging than the UK had expected when it agreed to it.
He and Mr Johnson have accused the EU of being inflexible.
The European Commission and member states say the UK should implement what it has negotiated, and that Brussels will use all tools at its disposal to ensure this happens. Under the terms of the agreement these could ultimately include trade sanctions.
The UK and EU have been locked in technical talks over the issue of Northern Ireland since the spring, and occasional high-level meetings between Lord Frost and his EU counterpart Maros Sefcovic have shown little progress.
Irish prime minister Micheal Martin said on Friday that a new veterinary agreement between the UK and EU could help remove the need for most checks under the existing framework.
He cited statements by Joe Biden's administration in the US that such treaty would not affect UK chances of negotiating a free trade agreement with the US.
But the UK is reticent to sign up to the Swiss-style “dynamic alignment” with EU rules Brussels says this would require, claiming it would be a breach of sovereignty.
The situation, under which disruptive checks are affecting trade across the Irish Sea, is expected to worsen significantly later this year when exemptions and grace periods end for supermarket suppliers.
The UK has already unilaterally extended the grace periods once, which the EU says is unlawful and a breach of the agreement.