Brexit will take years to negotiate and Britain can expect it be a very costly departure rather than a "cut-price" exit, Jean-Claude Juncker has warned.
The European Commission president spoke out amid unconfirmed reports suggesting the European Union will be seeking an exit payment worth £51bn over the next six years.
Brussels negotiators calculated the sum from the UK's share of projects and programmes it committed to as part of the 28-nation bloc, plus the cost of pension payments for EU officials who served during its 45-year membership.
The costly exit warning tempered more positive noises from Bank of England governor Mark Carney on the financial impact of Brexit.
Mr Carney, who previously issued dire warnings of a possible recession, told MPs: "There are scenarios where this process proceeds relatively smoothly to an increasingly clear end point and that will be consistent with a higher path for interest rates."
The governor said it would be reliant on Prime Minister Theresa May and her Brexit negotiators securing their aim of a "bold, ambitious trade deal".
Mr Juncker's statements to the Belgian Federal Parliament in Brussels underlined the size of the task though.
"Our British friends need to know - and they know it already - that it will not be cut-price or zero-cost," he said.
"The British will have to respect the commitments which they played a part in agreeing."
He then used a French slang term - "salty" - that translates as hefty or pricey.
"Therefore the bill will be - to use a rather vulgar term - very salty. It will be necessary for the British to respect commitments which they freely entered into."
Mr Juncker also reiterated that the price of exit must be decided before any future trade relations are discussed, denting the government's hopes of running the talks in parallel.
The Commission president said: "This will be a difficult negotiation, which will take two years to reach agreement on the exit arrangements.
"To agree on the future architecture of the relations between the UK and EU, it will need years."
Mr Juncker though played down claims of continental "hostility" to punish the UK for leaving as he paid tribute to British influence on the Continent.