"I would rather have preferred Britain not to have chose to leave the European Union, but anyone who leaves the European Union has to know, frankly, what this means", he said.
He also defended an European Union proposal for the bloc to continue to regulate trade in the British province of Northern Ireland after Brexit, should no other ways emerge to avoid a hard border between that territory and the country of Ireland.
If the European Union (EU) and Britain are to reach an understanding on their future relationship, London should provide more clarity on its view about it, Jean-Claude Juncker, president of the European Commission, told the European Parliament here Tuesday.
In her Mansion House speech on March 2, May said it was time to face the "hard facts" of Brexit.
Both Britain and the European Union have vowed to avoid the return of customs checks to the border and an interim deal in December left some flexibility on the issue, but an European Union text putting the agreement into law has sparked a fresh row with London.
He said it was "especially important" that Britain comes up with concrete proposals for the border between British-ruled Northern Ireland and the Republic of Ireland, which is staying in the EU.
Michel Barnier said any attempt by Britain to gain a competitive edge through the use of what is termed "dumping" would jeopardize hopes the country has for a smooth and orderly withdrawal from the EU.
The European Union wants the U.K.to promise to act "in full mutual respect and good faith", according to the first revision of the bloc's draft Brexit treaty. "If you decide to jettison, leave behind, the common agreements and rules, then you have to accept that things can not remain as they are".
The danger, Juncker claimed, was that the prime minister's goal of an "enduring" Brexit agreement would not be met, and the United Kingdom would be locked in constant and destabilising negotiations with Brussels for many years to come.
However, Prime Minister Theresa May has ruled out keeping the U.K.in an EU customs union because it would prevent the country from striking its own trade deals with fast-growing economies such as China and India.
In Britain, the financial services sector is expected to take the biggest hit by far, incurring about one-third of the extra red tape costs. "We see this with Brexit, we see this with trade, we see this across the board".