Jyrki Katainen, vice-president for jobs, growth, investment and competitive within the bloc, stressed the importance ensure negotiations lead to a “predictable” result for the sake of the European Union.
The Prime Minister will start the Brexit process when triggers Article 50 of the Lisbon Treaty on Wednesday – starting the formal negotiation stage.
She has previously signalled her intention to push towards a ‘hard’ Brexit – completely severing ties with the European project while working on the UK’s future relationship with the remaining 27 member states.
Mr Katainen, speaking at a Politico event in Brussels, told the audience it is “in everyone’s interest to avoid all of the instabilities” associated with Britain’s desired European split.
The ex prime minister of Finland warned against a 'hard' Brexit pursuit
That is why it is clear if that we are not successful we will have some unknown consequences
“Our main aim is to have negotiations which will lead to a predictable end – what ever it means,” he said.
“We don’t know what the predictable end is, but in Commission thinking it is in everyone’s interest to avoid all of the instabilities and risks.
“That is why it is clear if that we are not successful we will have some unknown consequences, and we all want to avoid the unknown consequences.”
The former Finish prime minister added: “The negotiations will be hard – technically, traditionally and economically speaking – nobody knows about the future arrangement between the EU and UK.
“Now it is time to put all of the emotions aside, and try to get as good and justified relationship as possible.”
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Speaking at the same event, Mario Monti, a former Italian prime minister, declared Brexit poses a “very serious threat”, adding David Cameron had made a “very bad choice” when opting to allow the referendum.
The Italian revealed he urged Mr Cameron to pursue a reform of the EU internal market, rather than immigration and benefits, as a more achievable goal during his ill-fated ‘renegotiation’ with Brussels.
He said: “It’s a very serious threat. I’m convinced it will show to have been a very bad choice for the UK, but a very unpleasant development for the EU.
“Not just politically and economically, but because of the flow of ideas. I for one will miss British influence in economic policy making in the EU. Britain had a rather strong leadership on the EU.”
Mr Monti added Mrs May could use Norway as a model if she makes a dramatic U-turn on her decision to have single market access.
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“Norway is a very decent country, it could be a model, it would have to be a model,” he said.
“They have been more coherent than the Brits in their unwillingness to be part of the EU in two referenda.
“But then have drawn the consequence in terms of accepting some more passive fruition of the single market, by not participating to the rule making, by complying with the existing rules made by others and by putting up an annual sum.
“There can be variations on that but I believe there ins’t too much room for imagination.”