The leaders of the European Union's three institutions warned Britain on Friday that it cannot expect "the highest quality access to the single market" unless it adopts the bloc's standards on environment, labour, taxation and state aid.
"Without being a member, you cannot retain the benefits of membership," they said in an op-ed published by several European media on Friday, ahead of Britain's exit from the EU at midnight in Brussels.
"Without the free movement of people, there can be no free movement of capital, goods and services," European Council President Charles Michel, European Parliament President David Sassoli and European Commission President Ursula von der Leyen wrote.
Union Flags line the Mall leading to Buckingham Palace in central London. AFP
The presidents were due to comment on Brexit and the future of the EU at 11 am. (1000 GMT).
Britain will no longer be an EU member state from Saturday, but it will enter a transition period until the end of December that is meant to give citizens and businesses time to adapt while an agreement is hammered out on the shape of the future relationship.
Trade, fishing rights and a raft of other issues are up for discussion.
Although the aim is to achieve a "zero tariff, zero quota" trade deal, the EU says that would also require "zero dumping."
It will insist that Britain remain aligned on standards and regulations to guarantee fair competition, but London has already indicated it will not be "a rule taker."