Theresa May has indicated she will fight a proposal to give EU citizens moving to the UK during the transition period after Brexit residency rights.
The PM, who is in China, said there had to be a “difference” between people from around the continent coming to the UK while it is still a member of the bloc and those who move after it exits.
She also sought to reassure Tory MPs worried over the length of transition.
Earlier this week, the EU set out what it was prepared to offer the UK.
The EU has said it foresees the transition lasting from the day of the UK’s departure on 29 March 2019 to 31 December 2020 but reports that it could be extended have dismayed some Brexit-supporting MPs.
Mrs May insisted such an “implementation period” will last about two years.
“We are not talking about something that is going to go on and on…We’re leaving the European Union. There is an adjustment period for businesses – and indeed government – for changes that need to be made,” she said.
In December, the two sides agreed a deal setting out the rights of EU citizens in the UK and British expats on the continent.
All EU nationals who have been in the UK for more than five years will be expected to be granted settled status, giving them indefinite leave to remain with the same access to public services as now.
Those who have been resident for a shorter period but who arrived before Brexit will be able to stay until they reached the five year threshold.
At the time, Downing Street said it envisaged anyone arriving after Brexit being able to continue to live, work and study in the UK but that they would need to register and the immigration details would have to be agreed as part of the wider transition negotiations.
The EU has since said it expects existing rules on freedom of movement – including the path to permanent residency – to apply in full until the end of the transition phase.
Mrs May, who is on the second day of a three day trade trip to China, said she would contest the issue of long-term residency rights when transition negotiations begin in earnest next month.
“When we agreed the citizens’ rights deal in December we did so on the basis that people who had come to the UK when we were a member of the EU had set up certain expectations,” she said.
“It was right that we have made an agreement that ensured they could continue their life in the way they had wanted to – now for those who come after March 2019 that will be different because they will be coming to a UK that they know will be outside the EU.”
“I’m clear there is a difference between those people who came prior to us leaving and those who will come when they know the UK is no longer a member.”
The BBC’s political editor Laura Kuenssberg, who is travelling with the prime minister, said Mrs May made clear that in the 2016 referendum vote people “did not vote for nothing to change”.
She said some of the business people travelling with the prime minister on the trip were optimistic about the reception in China, urging the government to help them “grasp the big opportunities” outside the EU.
But, she added, others warned of ongoing uncertainty with one senior executive saying “we are all wasting money” preparing their business in case a transition deal fails.
Another criticised the prime minister’s perceived lack of detail on her Brexit plan, saying, “the time was always going to come when there would have to be some decisions”.