Mr Barnier issued his warning the day after the UK and the European Commission agreed on the terms of a transition deal, hailed by Brexit Secretary David Davis as a “significant step”.
The EU negotiator’s call for caution was first issued yesterday when he reiterated some key outstanding issues remains to be settled.
Today, speaking to reporters, he said Brexit talks are far from over.
He said: “We are not at the end of the road.
“My experience of negotiations, even if this is an extraordinary and unique one, is that what remains at the end is often the most difficult.”
He highlighted that the UK and the EU still needs to agree on a number of burning issues, including the Irish border, data protection, geographical indications of various goods and the automatic recognition of each others’ court rulings.
Meeting in Brussels on Monday, Mr Davis and Mr Barnier agreed on the terms of a joint legal text according to which Britain will be able to negotiate and ratify trade deals with third countries from the very day it will officially leave the bloc although the UK will still be under EU regulations until December 2020.
The move was welcomed by the Federation of Small Businesses (FSB) National Chairman Mike Cherry, who called the agreement a “significal step forward”.
He said: “Finalising transition period terms will gift some certainty to the UK small business community and protect it from a damaging cliff-edge moment. Wherever possible, small firms want only one set of rule changes that take effect from the end of 2020.
“Confirmation that the UK will have the freedom to sign and ratify new trade deals during the transition period marks a critical step forward.”