Michel Barnier speaks during a press conference in Dublin Photograph:( AFP )
Britain will become the first country to leave the European Union on Friday, ending nearly five decades of membership.
Barnier, in Dublin for talks with Irish Prime Minister Leo Varadkar, said Britain had "no time to lose" as he warned of "the risk of a cliff-edge" change in relations at the end of the year.
"Round one is over and we have to rebuild everything," he told a joint news conference, referring to the end of the first stage in Britain's divorce from the European Union.
"If we have no agreement, it will not be business as usual and the status quo. We have to face a risk of a cliff edge, in particular for trade.
"We have no time to lose with any polemics, and to use this time to rebuild this friendship, this relationship."
Britain will become the first country to leave the EU at 2300 GMT on Friday, ending nearly five decades of membership.
An 11-month transition phase will then start, during which existing arrangements remain unchanged
British Prime Minister Boris Johnson wants to agree the terms of the future relationship, modelled on the EU's free trade agreement with Canada, by the end of that period.
Varadkar, who is fighting a general election on February 8, said Johnson's timetable would be "very challenging" but insisted the EU would enter the talks "united and in a positive spirit".
Barnier said he would present a draft negotiating mandate to the bloc's member states next Monday.
EU ministers are then set to agree their joint mandate on February 25 ahead of a meeting of the European Council in March.
No piecemeal solution
Varadkar argued the bloc was in "a very strong position" in the talks given the remaining 27 members' collective size compared to Britain.
"We have a population of 450 million people and the single market is the largest economy in the world," he said.
"But I don't think we have to see it as a contest."
In a separate BBC television interview broadcast on Monday, the Irish leader also warned Brussels would not accept anything less than a comprehensive free trade agreement with Britain.
London indicated earlier this month it could seek a piecemeal post-Brexit deal with the EU that leaves some issues unresolved but still lets it break free from the bloc at the end of the year.
But Varadkar said: "When I hear people talk about piecemeal, it sounds a bit like cake and eat," he said, referring to securing only a partial deal by the end of the year.
"That isn't something that will fly in Europe."