Boris Johnson has suggested the European Union has a legal duty under Article 50 to discuss a future trading relationship with Britain at the same time as working through so-called Brexit "divorce" issues.
The Foreign Secretary intervened after senior European figures, including EU chief negotiator Michel Barnier, voiced scepticism that talks would move on to future trade relations by the previously planned date of October.
The UK Government has been pushing to begin trade talks, arguing they are inseparable from the withdrawal issues which are currently being pored over by negotiators.
But the EU insists that "sufficient progress" must be made on the divorce issues - a financial settlement, citizens' rights, and the Irish border - before trade talks can begin.
Arriving for an informal summit of EU foreign ministers in Tallinn, Estonia, Mr Johnson said Article 50, which provides the framework for the exit of a country from the bloc, states that the two sides should discuss future relations.
"Article 50 makes it very clear that the discussion about the exit for a country must be taken in context with discussion of the future arrangements, and that's what we are going to do," the Foreign Secretary told reporters.
However, his comments came after Mr Barnier made clear that he was not yet in a position to declare that sufficient progress has been made on withdrawal issues to move on to future trade negotiations.
And other senior European figures expressed doubts that this stage will be reached by October, when London is hoping that leaders of the remaining 27 member states will give the green light to trade talks.
European Parliament president Antonio Tajani said he would advise the European Council to delay its assessment until December, while former Council president Herman van Rompuy said the chances of moving on to the second phase in October were "in the neighbourhood of zero".
Mr Johnson said he had "rock-solid confidence" in the Government's ability to get a good deal as he dismissed EU concerns over Britain's approach to the thorny issue of maintaining a soft Irish border after Brexit.
On Thursday, Mr Barnier fired a sharp rebuke at London as he called for a unique solution for the island of Ireland, accusing the Government of trying to get the EU to suspend its laws, customs unions and single market along the six counties of Northern Ireland.
"What I see in the UK's paper on Ireland and Northern Ireland worries me," he said.
Mr Johnson responded: "I think we can all work together to come up with a solution to that one.
"It is not beyond the wit of man. We have had a common travel area between the north and the south of Ireland for getting on for a century and we are going to continue to make that work."