The fund also warned that a turn toward protectionism in some advanced economies could threaten global prosperity.
Fears of foreign intervention in Western politics have heightened since the U.S. presidential elections in November.
While the U.K. has a larger military, Spain enjoys a proximity advantage.
If Spain grabs the Rock, Ceuta and Melilla will demand freedom from Spanish rule.
"The sovereignty of Gibraltar is unchanged and is not going to change," said Britain's foreign minister Boris Johnson.
EU leadership is determined to present a united front as Theresa May seeks deals for Britain.
"After more than 40 years of being united, we owe it to each other to do everything we can to make this divorce as smooth as possible," Tusk said.
The British people voted for Brexit partly because leaving the EU would let them 'take back control of our laws.'
The former prime minister resigned after failing to secure a Remain vote.
The European parliament has the power to veto a Brexit deal and will play a key role in talks.
A free trade deal like Canada has with the EU would not provide the same market access British firms currently enjoy.
EU member states are not a unified club and divisions are likely to emerge.
The letter gives us some clues about May's negotiating priorities.
The two sides will need to meet three key objectives by March 2019.
A board member of Germany's federal bank has warned Britain of the potential consequences of its exit from the European Union.
Can Britain's negotiations to leave the EU keep the spirit of free trade alive?
"If people want to have a bit of fun about how we dress, then so be it," May said.
Barnier warned the remaining 27 members of the EU would find it easier to cope with the fallout from such a scenario.
If Sinn Fein and the DUP do not form a power sharing government the U.K. may make the controversial move to rule from Westminster.
The prime minister will promote the 'great union of nations' of the United Kingdom in Scotland visit.