Brexit will still happen despite the UK becoming more aware of the "density of problems" during withdrawal negotiations, Jean-Claude Juncker has said.
The European Commission president said he disagreed with the Maltese prime minister Joseph Muscat, who reportedly said last week he saw "hopeful signs" that "Brexit will not happen".
His comments come amid increasing Tory tensions over the Brexit process, with Cabinet ministers publicly disagreeing over the potential terms of a transition phase after Britain's expected exit in March 2019.
Mr Juncker told Politico: "People will become more and more conscious of the density of problems on a daily basis, without always being able to provide a coherent answer to these problems."
But the Commission president said he still expected Britain to leave the European Union.
"I don't go as far as the Maltese prime minister who has not ruled out that it will not come to Brexit," he said. "My working hypothesis is that it will come to Brexit."
In a public Cabinet split last week, Chancellor Philip Hammond signalled that free movement of EU citizens would continue for a three-year transition period in all but name, with an added element of migrants having to register in the UK.
But International Trade Secretary Liam Fox insisted unregulated free movement of labour after Brexit would "not keep faith" with the EU referendum result and that the Cabinet had not agreed a stance on immigration
Shadow Brexit secretary Sir Keir Starmer has suggested Labour could seek to exploit the Tory splits, saying the party will "work with others" to ensure a transition phase including membership of the single market and customs union.