Boris Johnson has failed in his bid have a Brexit agreement overhauled, despite a call with European Commission president Ursula von der Leyen aimed at convincing her to renegotiate.
The prime minister wants significant changes to the Northern Ireland protocol, an agreement with the EU which sees some checks on goods moving across the Irish Sea.
Requests for a renegotiation on Wednesday were rejected, leading the prime minister to call Ms von der Leyen personally in an attempt to win her over.
But the EU chief tweeted following their call saying she had rejected his request.
She wrote: "PM Boris Johnson called to present the UK Command paper on the Irish/Northern Irish Protocol.
"The EU will continue to be creative and flexible within the Protocol framework. But we will not renegotiate.
"We must jointly ensure stability and predictability in Northern Ireland."
Downing Street said the prime minister told Ms von der Leyen the protocol is "currently operationally unsustainable" and urged the European Union to engage with solutions put forward by the UK.
Asked about Ms von der Leyen statement she would not renegotiate, a spokesman for the Prime Minister said the Government wanted to be "constructive and collaborative" in a bid to mend the current issues.
Earlier in the day, Business Secretary Kwasi Kwarteng said the protocol, which effectively keeps Northern Ireland in the EU's single market for goods, was never "something that was going to last forever".
He told Sky News: "A deal is a deal but it wasn't something that was going to last forever.
"Nobody thought the Northern Ireland Protocol was going to define the role of Northern Ireland within the UK forevermore, it was something that was flexible.
"You'll remember two years ago people said we were never going to get a deal from the EU but we did so.
"When people say they're not going to look at the protocol again, I say 'well, let's just see'."
On Wednesday Brexit minister Lord David Frost told peers the UK "cannot go on as we are" regarding the Northern Ireland protocol.
He suggested changes to the protocol, which include:
- An "evidence-based and targeted approach" to goods at risk of entering the single market, but products destined just for Northern Ireland would be allowed to circulate "near-freely".
- Continued access in Northern Ireland to goods from the rest of the UK, through a regulatory approach which accepts both British and European Union standards.
- A "normal" treaty framework to govern the arrangements, with no role for the Court of Justice.