"It's a historic day that will mark Europe's starting down a road that it not well known," he told journalists after an event organised by the financial news agency Bloomberg.

"We hope there will be an understanding," he went on. "The negotiations must have an end that is cooperative. What we want is that European citizens can end up benefiting from this change in the context in which it is taken, because political decisions are only guided by that motive.

Centeno, who had travelled to London mainly to meet investors, argued that the EU's economic situation in terms of public sector budgets and external accounts is positive compared with "the imbalances that exist in the US" or even in the UK itself.

"The European economy is a very strong economy that has a very, very strong domestic market and which is benefiting from a range of reforms," he said. "This gives the euro zone a power that it must use at a moment when it must meet the challenges not only of Brexit, but also of protectionism from the US administration, which can bring greater difficulties."

An envoy of the UK government earlier handed over to the president of the European Council in Brussels the letter formally triggering Article 50 of the Treaty of Lisbon, under which a member state can leave the Union. That follows the referendum of June last year in which a majority voted to leave.

Under the treaty, the two sides have two years to reach agreement on the terms of the departure. If none is reached, and all member states do not agree to extend the deadline, the departing state leaves anyway.