The requirement is part of the draft guidelines released by Donald Tusk, the EU Council president, in reply to Theresa May's Article 50 notification letter.
Draft guidelines obtained by The Associated Press say the European Union and Britain must first "settle the disentanglement" of Britain from the bloc.
Johnson, speaking in Brussels upon arrival for a North Atlantic Treaty Organisation meeting, said he has had good feedback from partners since Wednesday's British formal announcement of its departure from the European Union, despite worries on both sides of the Channel about Brexit.
If the guidelines are adopted, the talks are certain to be hard, with both sides already in disagreement on issues from the sequence of the talks to the conditions Britain would have to accept to secure a post-Brexit transitional deal.
The European Union (EU) demanded that Britain make "sufficient progress" on its divorce before talks on a trade deal can start, as it laid out its tough Brexit negotiating plans yesterday.
The U.K. government on Wednesday officially began the process of leaving the European Union, more than nine months after the electorate voted for Brexit.
"It is clear both sides wish to approach these talks constructively, and as the Prime Minister said this week, wish to ensure a deep and special partnership between the United Kingdom and the European Union", a United Kingdom government spokesman said.
Mr Tusk's negotiation guidelines are expected to endorse the leaders' insistence that Brexit terms - including the so-called "divorce bill" - must be settled before a new trade relationship can be discussed. If negotiations are not completed within that timeframe, it can only be extended by unanimous consent of all remaining member states.
In a referendum centered on uncontrolled immigration and national self-determination, Great Britain made a decision to leave the European Union on June 23, 2016, the first EU member state to ever do so.
The European Parliament said Britain would violate law if it negotiates trade deals with individual companies before it leaves the EU.
The European Parliament wants the rights of Europeans in Britain and of British citizens in Europe to get top priority in Brexit talks. This is why I rule out this kind of interpretation. that security cooperation is used as a bargaining chip.
He said it would also mean that workers' rights, environmental protection and consumer rights now enshrined in European Union laws would continue as United Kingdom laws - although Parliament would be free to change them later.
Negotiations will only be able to move to the next phase once member states agree that sufficient progress has been made, and an agreement on the arrangements for an orderly withdrawal has been reached.
You can play Ms. Pac-Man in Google Maps
The icon pops up in the lower left corner, and the map zooms in to the correct size to play, but nothing actually shows up. We're not quite sure what the minimum is, but we imagine most of you will be able to load a game without issue.