The Prime Minister has signalled that freedom of movement could be part of a transitional agreement with the European Union if separation talks aren't concluded within the two-year timescale.
Theresa May said there would need to be an "implementation period" to help businesses prepare.
Speaking on a trip to the Middle East, May was asked whether EU citizens should still have a right to come to the UK and vice versa, during the uncertain interim period.
"Once we've got the deal, once we've agreed what the new relationship will be for the future, it will be necessary for there to be a period of time when businesses and governments are adjusting systems and so forth, depending on the nature of the deal, a period of time during which that deal will be implemented," May said.
A temporary arrangement could take effect from March 2019, when the UK has officially withdrawn from the EU, and would then be in place until a permanent trade deal is struck between the UK and the bloc.
European council president Donald Tusk has said that while the member states will be open to seeking transitional arrangements, the "core principles", including regarding immigration, will have to be maintained during that period.
His comments suggest that if the UK wants to stay in the Single Market while a free trade is negotiated and then brought in past the two-year formal exit timeline, then free movement will have to remain.
Tusk has also said EU leaders won't pursue a punitive approach as "Brexit in itself is already punitive enough".
The Prime Minister has though, also noted that she had been sent a clear message by the Brexit vote to put control of immigration back in the hands of UK government.
"What is crucial for the British public, what was part of the vote that they took last year, was that they want to ensure that we have control of our borders and control of our immigration and that's exactly what we will do when we come out of the European Union," she said.