Britain has said it will fight back if the EU will not strike an acceptable exit deal.
With tensions high as next month's Brexit talks approach, Britain has said it will fight back if the EU will not strike an acceptable exit deal.
This has sparked fears in Brussels that London would attempt to undercut EU regulations after the divorce on a range of issues if talks turn sour.
"The UK must be aware that any free trade agreement will have to ensure a level playing field and encompass safeguards against unfair competitive advantages," Tusk told MEPs at the European Parliament in Strasbourg, France.
This guarantee would involve a wide range of matters, including "tax, social, environmental and regulatory measures and practices," Tusk said.
Negotiations on a trade deal are not expected until talks on the fate of the millions EU citizens and Britain's exit bill have made "sufficient progress".
Tusk hailed the EU's remaining 27 members who "have been united, consistent and have demonstrated solidarity with one another since the Brexit referendum last June."
The Brexit negotiation "is now in the hands of our capable divorce lawyers," said European Commission head Jean-Claude Juncker, who also addressed MEPs, in reference to the EU's chief negotiator Michel Barnier.
"I want to wish him luck as I know he will leave nothing to chance," Juncker said.