The Prime Minister appeared to concede that a trade deal with the EU cannot be signed until after the two-year Article 50 process is completed.
Speaking during a visit to Jordan, Mrs May said she expected an outline of the future trade relationship to be clear to everybody by Brexit Day in March 2019.
But the PM appeared to accept that the signed deal will have to wait until after the UK has left the bloc.
European Council president Donald Tusk left no doubt in his draft guidelines for negotiations, released on Friday, that EU rules state that trade deals can only be done with non-members.
He said the most the UK can hope for prior to Brexit is an "overall understanding on the framework for the future relationship".
Mrs May insisted it will be possible to reach clarity on the deal within two years.
But asked if the deal could be finalised in that timescale, she told Sky News: "There's obviously a legal situation in terms of how the EU can conduct trade negotiations.
"I'm clear that by the point at which we leave the EU, it's right that everybody should know what the future arrangements, the future relationship, that future partnership between us and the European Union will be.
"That's the sensible thing, it's the pragmatic way to look at this, and I believe that's what we will do."
Labour said that Mrs May's comments amounted to a "significant retreat" from the Government's previous position that a trade deal could be done within the two-year deadline for withdrawal negotiations set down in Article 50 of the EU treaties.
The party's Brexit spokesman Paul Blomfield said: "It is less than a week since the Prime Minister triggered Article 50, and it seems every day brings another broken promise from the Government.
"First they said immigration may go up after Brexit. Now they are backpedalling on trade deals.”
He said Labour would hold the Government to account on the pledges made to the British people during the referendum campaign and since.
Mr Blomfield added: "Now, as they face reality, they are trying to downplay expectations. They need to spell out the transitional deal that will be in place, to stop the economy falling off a cliff edge without new agreements in two years' time."
Labour MP Owen Smith, a leading supporter of the Open Britain campaign, said: "Bit by bit, the main planks of the Prime Minister's Brexit strategy are falling away. Today, she has admitted that we will not have the time to agree a trade deal with the EU before the Article 50 period is finished."
Leaving without a trade deal would mean "our economy will go off a cliff edge, hitting our businesses with punishing tariffs and putting jobs at risk", said Mr Smith.
He added: "Ministers have to prevent this hardest of hard Brexits by at least agreeing a transition deal with the EU that would keep us in the single market before a new trade deal can be signed."