By Laurence Norman and Jenny Gross 

BRUSSELS -- The European Union's top official said Tuesday the U.K. would face a steep bill on leaving the EU and warned it would take years to negotiate future EU-U.K. economic ties, marking some of the toughest comments yet delivered by a senior European politician.

"The bill will be, to put it somewhat crudely, very steep," European Commission President Jean-Claude Juncker said in a speech to the Belgian Parliament, without giving specifics. He also predicted the negotiations would be "difficult."

Roughly a month before U.K. Prime Minister Theresa May is expected to trigger a two-year window for negotiations, Mr. Juncker's comments suggest a hardening of positions ahead of the talks, even as both sides say they want an amicable divorce. British officials and EU officials have signaled fundamental differences on key points and even over the process, paving the way for a contentious start.

A U.K. government spokesman on Tuesday declined to comment specifically on Mr. Juncker's comments but said Britain was confident that it could get a good deal from the EU.

"We haven't begun negotiating our exit yet," the spokesman said. "We're approaching negotiations in a mature and open way in the spirit of goodwill.

While EU officials maintain that determining the terms of the future relationship between the U.K. and the bloc will be lengthy, British officials say they hope to end the two-year Brexit period with a deal not only on exit terms but also on their future trade and economic ties.

Mr. Juncker said Tuesday that in addition to the two-year window to agree upon "the future architecture of the relationship between the U.K. and the European Union, we will need some years."

European officials have said in recent weeks that the EU will present Britain with a bill of EUR55 billion to EUR60 billion ($58 billion to $63 billion), which covers past British spending commitments that will come due in the future on various items, including economic assistance to Eastern European countries and sharing the pension bill for EU employees.

The European Commission, the EU executive, will conduct the day-to-day negotiations with the U.K.

U.K. officials, including Foreign Secretary Boris Johnson, have argued that discussions on a future trade deal could take place in parallel with the exit terms and wrap up soon after.

David Davies, a Conservative lawmaker who backed leaving the EU, said Mr. Juncker can throw demands around, but the EU can't force the U.K. to act in a way it wants.

"What are they going to do if we don't pay?" Mr. Davies said. "National governments contain politicians who have to win elections, and they're not going to be able to do that if they cut off their largest market from their manufacturing sectors."

Write to Laurence Norman at laurence.norman@wsj.com and Jenny Gross at jenny.gross@wsj.com

(END) Dow Jones Newswires

February 21, 2017 14:48 ET (19:48 GMT)