The United Kingdom Parliament ratified Johnson's withdrawal agreement with the EU on Wednesday, paving the way for Britain to leave the EU.

However, the European Union immediately warned the UK that its negotiations will now get much tougher after Britain leaves the EU.

Negotiations over Britain's future relationship with the EU will not begin until Britain has left.

Johnson has just 11 months to negotiate a new deal or risk a new cliff-edge Brexit.

Visit Business Insider's homepage for more stories.

The European Union has warned the United Kingdom that negotiations over a Brexit trade deal will b "more difficult" than talks over the Brexit divorce deal, which took years for both sides to thrash out.

The UK Parliament voted on Wednesday to ratify Boris Johnson's Brexit agreement, paving the way for Britain's exit from the EU at the end of January.

However, negotiations over Britain's future relationship with the EU will not begin until after the country has left.

Stefan de Rynck, a senior aide to the EU's chief negotiator Michel Barnier, said on Wednesday that "there should be no misunderstanding" of how complicated upcoming talks over a potential UK-EU free trade deal would be.

In comments reported by Politico, he said the UK needed a "dose of realism" about how much could be achieved by the end of 2020, the deadline Boris Johnson has set himself for negotiating a full, comprehensive free trade deal with Brussels.

"There should be no misunderstanding of the fact that the next phase will be more complicated to negotiate than the Withdrawal Agreement," he said.

"The construction of the text for the Withdrawal Agreement wasn't always easy - but compared to the construction of the text for the future relationship agreement, we are talking about two different kinds of exercises."

"The limitation of time must lead to some dose of realism on what can be achieved."

De Rynck's comments are a direct riposte to the prime minister, who has insisted it is "epically likely" that a full and comprehensive deal can be achieved by the end of the year when the transition period ends.

De Rynck said that when both sides initially envisioned the transition period as lasting 21 months.

But MPs' repeated rejection of the Withdrawal Agreement Bill means the time available is now closer to 11 months.

He said the EU thought that even "21 months was a rather short transition period in terms of the negotiations that need to be done."

De Rynck's intervention came after the Commons passed the Withdrawal Agreement Bill — which sets out the terms of the UK's divorce from the EU — in parliament on Wednesday night.

The legislation will now receive Royal Assent and pass into the statute books, bringing to an end nearly three years of parliamentary warfare which saw it rejected on three previous occasions.