MEP Helga Stevens told a European Parliament Presidential debate her “red line” would be “if it’s better of the UK to leave than stay in”.
The European Conservatives and Reformists Group’s Presidential candidate said the deal needs to be one which “benefits both of us”.
She told the debate, organised by Politico: “If it’s better for the UK to leave than stay in, then that’s my red line.
“My belief is what we need to do is a fair deal which benefits both of us.
“The UK will still exist, still be part of Europe so we need to make an effort to make sure there’s a win-win situation.”
The 48-year-old Belgian was joined by six other MEPs, including Romanian politician Laurentiu Rebega who said the EU still does not understand Brexit.
He told the panel: “We have to understand why Brexit happened and why it is like this.
“We are still going in the wrong direction to understanding.”
The EU’s Brexit negotiator, Guy Verhofstadt, set off warning lights in Britain after he told the panel it would be tough to give the thumbs up to a deal the EU would benefit from.
He said: “Parliament will indicate what red lines will be in the negotiations.
“It’s going to be tough and also very hard to have a green light.
“Parliament has to be linked to these negotiations immediately and not be pushed aside.”
He also struck upon one of the main arguments surrounding Brexit – the issue of the freedom of movement within Europe.
The former Belgian Prime Minister, also agreeing with Ms Stevens, added: “We need the four freedoms, inseparable, no question about freedom of movement.
“It cannot be a situation about you’re better out than in.”
And showing a glimmer of hope for Remain advocates, he said EU citizenship could not be taken away from the British – although did not explain how this could possibly work after Brexit.
He said: “Last thing, we cannot take away all EU citizenship from Brits.”
In November former European Parliament president Pat Cox warned European countries could block the UK’s attempts to get a favourable post-Brexit trade deal.
He said the UK needs all members to approve Brexit deals but some countries could refuse to ratify a deal, putting Brexit at a standstill.