Theresa May says she wants a deal to guarantee the status of expats "early on" in Brexit negotiations.
In a statement to MPs, Mrs May rejected requests to reveal more about the government's intentions for negotiating the UK's departure from the EU.
But she suggested the government would publish a Brexit plan "some time" before triggering formal talks.
Labour's Jeremy Corbyn said the "shambolic" government was producing "mixed messages" on Brexit.
The PM was addressing MPs about last week's EU summit, in which the other 27 EU leaders met in her absence to discuss Brexit.
She promised a "calm and measured approach", adding: "It remains my objective to give reassurance early on in the negotiations to EU citizens living in the UK and UK citizens living in EU countries that their right to stay where they have made their homes will be protected by our withdrawal."
The government has faced calls from across the House to guarantee the right of EU nationals to remain in the UK after Brexit.
But it insists this must form part of a reciprocal arrangement also protecting the status of UK expats living in other European countries.
The PM also said she would start the UK's exit from the EU on time, regardless of next month's Supreme Court verdict on whether Parliament must be consulted.
She has promised to give notice of the UK's departure under Article 50 of the Lisbon Treaty before the end of March.
And she did not rule out the UK continuing to pay contributions to the EU after leaving.
Responding to Conservative MP Philip Davies' request for a "pledge" that no more contributions would be made, she said: "When we leave, people want to ensure that it's the British government that dictates how taxpayers' money is spent."
The government has agreed to publish a "plan" before formal talks begin, although it has not said how detailed this will be.
The PM replied "yes" when Labour's Angela Eagle asked whether this would be released "some time" before Article 50 is invoked.
Mr Corbyn said members of the government front bench were giving "mixed messages" on the UK's departure from the EU, including on whether a transitional deal will be needed to cover trade arrangements before all the talks have been completed.
There are a "babble of voices speaking for themselves and their vested interests" the Labour leader said, asking the PM to "represent all sides, whether they voted to leave or remain".