The EU commission chief also stated Brexit was a “tragedy” as leading eurocrats met to celebrate the milestone in Rome.
Speaking to reporters, Mr Juncker said: "In fact, it is a very sad moment, I do think that the Brexit, the exit of Britain, is a tragedy."
Asked how the EU would continue without the UK, he added they would have to do it as "a 27" member bloc.
Jean-Claude Juncker said it was "a very sad moment" the UK was absent from the Rome celebrations
In fact, it is a very sad moment
Jean-Claude Juncker made the comments as he arrived at Saturday’s anniversary ceremony, which marked the 60th anniversary of the agreement between six founding countries to create the European Economic Community, the forerunner of the European Union.
Earlier this week, just three MPs spoke at a poorly-attended debate in London to mark the occasion.
The SNP’s Stephen Gethins, who secured the Westminster Hall debate, urged MPs to remember the “unprecedented success” of the bloc since the signing of the Treaty of Rome on March 25, 1957.
Brexit: Results of how the UK voted Mon, March 20, 2017
Much of the North East of the United Kingdom voted to leave the European Union including Sunderland, Gateshead, Darlington, Durham, Hartlepool, Middlesbrough, Stockton, Redcar and Cleveland, North Tyneside and South Tyneside, and Northumberland
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GREAT YARMOUTH: The town of Great Yarmouth on the East Coast of England voted by 72% to leave the European Union.
Referring to the Leave vote in June last year, he said: “At this time of uncertainty in terms of our relationship with our European partners, I think it's easy to lose sight of the major contribution EU membership has for us all.”
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The move will kick-start the beginning of the two-year countdown until Brexit is complete.