European Council President Donald Tusk warned Britain Wednesday that leaving the EU without any formal agreement would harm the U.K. most.

With Britain set in coming days to launch two years of negotiations preparing its departure, Tusk responded to "threats" from across the English Channel that it might be best to walk away without a deal.

"I want to be clear that a no-deal scenario would be bad for everyone, but above all for the U.K., because it would leave a number of issues unresolved," he told EU lawmakers in Strasbourg, France.

"We will not be intimidated by threats, and I can assure you they simply will not work," he said.

The people of Britain voted in June to leave the EU — the first member state ever to do so — in part out of concern about migrant workers from other member states exploiting the U.K.'s relatively generous social welfare scheme, and worried by the country's apparent loss of sovereignty to an unelected EU bureaucracy in Brussels.

British Prime Minister Theresa May has promised to trigger the process for leaving the 28-nation bloc by the end of the month. That would launch two years of talks that would see the country exit early in 2019.

"We will have no choice but to start the withdrawal talks once the UK notifies," Tusk said. "It is our wish to make this process constructive, and conducted in an orderly manner."

That means addressing the uncertainty of around 3 million EU citizens living in the U.K., and 1 million Britons who reside in the 27 other nations. A major battle looms over money. Britain could face a hefty divorce bill of up to 60 billion euros ($64 billion), but while it hasn't refused outright to pay, London does object to that sum.

Once May triggers the EU exit clause — Article 50 of the bloc's governing treaty — European officials would require about a month to organize a summit of EU leaders and finalize negotiating guidelines and a mandate for Michel Barnier, a former French government minister who is leading negotiations on behalf of the EU.

The negotiations, which would also outline the future shape of Britain's relationship with the EU, would have to be concluded by October 2018 to leave time for European, and possibly British, institutions to adopt the final agreement.