David Frost told a gathering of the UK’s governing Conservative Party that the Brexit agreement, which he negotiated and was signed by Britain and the EU, was undermining peace in Northern Ireland and causing “instability and disruption”.

He said unless there are major changes to the deal, Britain will invoke Article 16, a provision that lets either side suspend the agreement in exceptional circumstances.

However, Britain has made that threat before, and Frost didn’t pull the trigger. “But we cannot wait forever,” he said.

The crisis straining UK-EU relations stems from trade arrangements for Northern Ireland, the only part of the UK that has a land border with the 27-nation bloc. The divorce deal the two sides struck before Britain’s departure means customs and border checks must be conducted on some goods moving between Northern Ireland and the rest of the UK.

The regulations are intended to prevent goods from Britain entering the EU’s tariff-free single market while keeping an open border between Northern Ireland and EU member Ireland, a key pillar of Northern Ireland’s peace process. But the checks have angered Northern Ireland’s unionists, who say they impose burdensome red tape on businesses and weaken Northern Ireland’s ties with the rest of the UK.

Britain accuses the EU of being needlessly “purist” in implementing the agreement, known as the Northern Ireland Protocol, and says it requires major changes to work.

Blaming the bloc’s “heavy-handed actions,” Frost said “the Protocol itself” was undermining Northern Ireland’s 1998 peace agreement. “Businesses, political parties, the institutions, and indeed all in Northern Ireland face instability and disruption,” he said.

“So I urge the EU to be ambitious. It’s no use tinkering around the edges. We need significant change.”

Officials in the EU are likely to greet Frost’s ultimatum and his description of Britain’s EU membership as a “long bad dream with disdain”.

The EU blames Britain for the disruption, accusing it of trying to renege on a legally binding deal it signed up to.

Britain has also angered the EU by delaying the introduction of checks on some goods from the bloc agreed to in the divorce deal, citing the burden on businesses.

Frost said Britain was awaiting the bloc’s response to its proposals for changes. European Commission spokesman Daniel Ferrie said the bloc would propose solutions soon. “What we’re focusing on is finding practical solutions to issues that are arising, nothing more and nothing less,” he said.

Britain’s Conservatives are holding their first in-person conference since Britain left the bloc last year. Despite Prime Minister Boris Johnson’s vow to “get Brexit done,” the rupture is roiling the economy as Britain tries to bounce back from pandemic disruption and damage.

A severe shortage of truck drivers, due in part to a post-Brexit exodus of European workers, has snarled British supply chains, leaving gaps on supermarket shelves, fast-food chains without chicken and gas pumps dry. Scores of soldiers have been drafted in to drive fuel tankers after more than a week of gas shortages.