A senior ally of Emmanuel Macron has told Brexiteers that they must "face reality" over the consequences of leaving the EU.
Nathalie Loiseau, who on Monday led the first European Parliament delegation to the London since Brexit, said those "complaining" about the deal Britain signed had failed to propose alternatives.
Ms Loiseau, a former French European minister who represented Mr Macron's government during talks, said issues on the Northern Ireland border were an inevitable consequence of leaving the EU and could only be "mitigated".
Her comments come days after the Democratic Unionist Party – which opposes the Irish border agreement – refused to return to a power-sharing government in Northern Ireland, citing opposition to the Brexit deal.
British MPs and members of the European Parliament from across the EU's 27 member states met in Westminster on Monday for a joint parliamentary assembly, where they discussed issues ranging from the energy crisis to the loss of free movement.
Maroš Šefčovič, the EU's Brexit chief, used a speech at the meeting to urge Rishi Sunak to "abandon" Boris Johnson's Northern Ireland protocol bill – which if passed would unilaterally overwrite parts of the agreement to better suit the UK.
The EU says the bill would break international law but UK ministers believe it is necessary to meet the concerns of many unionists, who have criticised the agreement for putting checks on trade between Great Britain and Northern Ireland.
Mr Šefčovič, a vice president at the European Commission, warned that "unilaterally disapplying core parts of the Protocol" with the bill would also have "serious consequences for our trade relationship".
MEPs attending the meeting were even more blunt. Irish MEP Seán Kelly, from Ireland's ruling Fine Gael party, said pushing ahead with the bill would start a "trade war".
"If the bill were to go through and become law here, then that would absolutely be chaotic from a European point of view, because if would actually lead to a trade war," he told reporters ahead of the summit.
Speaking at the same press briefing, senior Conservative MP Sir Oliver Heald, who leads Britain's delegation, said the bill was clearly "not a solution" to the current deadlock – though he stopped short of saying the government should scrapping it.
"I think the answer is to get an agreement ... they are talking now, but we need them to redouble their efforts," he said.
"If we could sort out the original problem in Northern Ireland with the Northern Ireland Agreement, we ought to be able to sort this out.
"And it's very important for Ireland and Northern Ireland and for us, because there's so much we could do to cooperate effectively in all our interests.
"That's what I would like to see. So I think the bill is, in a way, not the solution. The solution is an agreement."
French MEP Ms Loiseau said she did not want ignore concerns of the unionist community, "provided that people will come with solutions". But he said these had so far not been forthcoming.
"I asked many times when we were in Belfast okay, if you don't like the protocol, what's what's the alternative? What's your solution? And there was no answer," she told reporters.
"And maybe the question goes back to Brexit itself. Brexit does have consequences that can only be mitigated, but cannot be erased by themselves.
"Of course, if you have been complaining, saying that Brexit was the solution, and that the protocol was a problem, at some point you find you have to face reality."
The UK-EU parliamentary assembly was established by the Brexit agreement to scrutinise the UK and EU's approach to implementing the deal.
Delegations consist of 35 MPs and peers and MEPs each. It has powers to ask questions of ministers and the Commission, and to demand information or documents relating to the implementation of the deals.
