In an outspoken intervention Catherine Day, who was once the bloc’s most senior official, said that an agreement will be reached between the two sides “unless the madmen intervene”.
She branded Theresa May’s threats to walk away from the negotiations “hollow” but anticipated the talks will go down to the wire as both sides play chicken over the possibility of a no-deal scenario.
Instead, she said the UK will mostly likely end up with an agreement similar to Norway, which is in the Single Market but not the Customs Union, but with “a little bit more trimmings at the margins”.
The comments by Irish official Ms Day, who is one of five special advisors to the powerful EU Commission boss, are likely to anger eurosceptics who feel some in Brussels are still trying to keep Britain in the club.
Speaking on the sidelines of a conference in Dublin earlier today, she told Bloomberg: “It will be late and messy because that’s when you get minds to concentrate.
“The EU isn’t going to go through the pain of negotiating a whole new model. It’s going to say if you want to stay in the single market or customs union, we have this to offer you.
“If you don’t, then it’s free trade plus and Canada is our most developed version. They say ‘we’ll leave with no deal’ and we are supposed to be on our knees. No. I think the British will be pragmatic in the end.”
Norway participates in the Single Market through the European Economic Area (EEA) agreement, which gives it full access in return for accepting the rules without having a say in how they are made.
The European Free Trade Association (Efta) countries are, however, exempt from the bloc’s controversial agriculture and fisheries schemes and have full control over their own trade policy.
Proponents of a softer Brexit advocate this model as a way of fulfilling the referendum vote to leave the EU but heeding business concerns by remaining inside the Single Market.
However, eurosceptics argue that it would be unacceptable for Britain to become a rule taker and point out that Efta states have to accept free movement and indirect ECJ jurisdiction.
In contrast, Canada has just sealed one of the most comprehensive Free Trade Agreements (FTAs) in history with the EU, which removes 99 per cent of tariffs on goods between the two sides.
However the deal is much more patchy in the crucial area of services – which make up most of the British economy – and provides limited protection for key sectors like the City of London.
Ottawa has not had to accept any of the conditions of EU membership, with its trade minister having observed that Canada now feels like “part of the Common Market” – the precursor to the more integrated Single Market.
Ms Day’s remarks came as it emerged that a group of senior Tory MPs are planning to go over Theresa May’s head and seek their own Brexit talks with the European Union.
Top conservatives, including former ministers, want to establish their own communications line with Brussels so they can organise resistance in Parliament to a hard Brexit.