The EU Commission president raised the possibility that the UK will carry on being full members of Europe's political institutions even after official negotiations on divorce proceedings have been concluded.
Strikingly a markedly conciliatory tone the Brussels boss insisted he wanted a "fair deal" with Britain which was in the best interests of both sides, and revealed he had told Theresa May there was no "hostility" towards the UK from within the EU.
Jean-Claude Juncker hinted that the EU could seek a transitional deal with Britain
But it was his unguarded remarks about the possibility of British MEPs taking part in the next European Parliament elections, which are slated in for after the UK's official departure date from the EU, which will get chins wagging in Whitehall.
The next European Parliament elections are scheduled for May 2019 and they only take place once every five years, raising the prospect of the UK remaining tethered to the EU until 2024.
The EU chief said British MEPs could stay in the European Parliament beyond 2019
Mr Juncker made the remarks at a press conference alongside Maltese PM Joseph Muscat
During a press conference at the parliament building in Strasbourg this lunchtime the chief eurocrat was grilled by journalists about his response to Theresa May's landmark speech yesterday, in which she pledged to make a clean Brexit and pull Britain out of the single market.
And whilst congratulating his colleague Antonio Tajani, the Italian MEP who is the new EU Parliament president, he let slip that British politicians could be involved in the Brussels club for many years to come.
He said: "Once again Antonio, all the best for your mandate which will be up to the next European elections without or with the British members of the parliament, this has to be seen."
Mrs May has promised to trigger Article 50 by the end of March, meaning that Britain is set to leave the club by the same date in 2019. The next European Parliament elections are not scheduled to take place until May 23 that year.
Elsewhere Mr Juncker stuck to the mantra that membership of the single market comes with "obligations", including accepting the free movement of people, but struck a sober and conciliatory tone towards Mrs May.
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We want a fair deal with Britain and a fair deal for Britain, but a fair deal means a fair deal for the European Union too
He also revealed some details of her phone call with the PM yesterday evening, telling reporters: "I was repeating once again that we as a Commission and Michel Barnier as our chief negotiator, we're not in a hostile mood.
"We want a fair deal with Britain and a fair deal for Britain, but a fair deal means a fair deal for the European Union too.
"I'm quite satisfied that she was taking away from this unclear landscape some elements making it clearer, but our basic position is the same. We will start to negotiate after the triggering of Article 50 and then we'll see.
He added: "It will be a very, very, very difficult negotiation because Britain has to be considered as a third country which I'm not used. I'm unhappy about this, but that's the situation we are in and we'll make the best of it."
Also at the press conference was the Maltese prime minister Joseph Muscat, who had earlier dismissed parts of Mrs May's Brexit plan as "detached from reality", but here was considerably softer towards Britain.
Theresa May's 12 point Brexit plan
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Continued cooperation with the EU to tackle terrorism and international crime.
He revealed that eurocrats are largely unperturbed by a perceived threat the PM made towards the EU in her speech, and said they were pleased that her remarks had finally brought clarity on what Britain wants.
He said: "Reading the British press this morning, I did not see in the prime minister's words when I heard her first hand the sort of declaration of war that some media are depicting it was.
"I saw this as a statement of clarity on which one might have reservation, but I actually thought it was very welcome that rather than saying that we want to stay in the single market but want exception because our country is different from the others.
"Abandoning that sort of view and saying we're going to withdraw from the single market because we want to prioritise the control of people coming over from Europe, I respect that because it respects our position."