The EC chief told the BBC that the Brussels would negotiate with the UK in a friendly and fair way but that a bill would have to be paid - but the final amount had not been precisely calculated yet but that it was "around" £50bn.
"The British government and parliament took on certain commitments as EU members and they must be honoured. This isn't a punishment or sanctions against the UK," he said.
Part of the reason seemed to be the prevention of other countries from following the UK lead.
Juncker insisted the 4.5m EU citizens living in the UK and British citizens currently living across the EU would not be forced to leave.
Prime Minister Theresa May has pledged to launch the two-year process of negotiations to leave the EU on 29 March by triggering Article 50 of the Lisbon Treaty.
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