EU nationals living in the UK will keep all of their current rights until the country leaves the union, a senior official has said.
Mark Sedwill told MPs "there will be no knocks on the door" when asked about uncertainty following the Brexit vote.
Britain is expected to seek to introduce controls on free movement rules but the details of the system are yet to be outlined.
Ministers have been urged to provide a firm guarantee about the status of EU citizens already in the country ahead of negotiations with Brussels, which will also cover the rights of Britons living in other member states in the bloc.
The Government has said it "fully expects" that the legal status of EU nationals living in this country, and that of UK nationals in EU member states, will be "properly protected" when the UK leaves.
Mr Sedwill, the Home Office permanent secretary, told the Commons Home Affairs committee: "Until the UK leaves the EU all EU citizens have exactly the same rights they had on the 22nd of June (the day before the referendum).
"Those rights include, if you have been here for five years, the right to permanent residence and if you've been here for six years... the right to apply for citizenship if you wish to do so. I see no circumstances in which those would change."
Mr Sedwill said the Government's position is that all of Britain's rights and obligations of membership continue until it leaves the EU, adding that "there is clearly a question beyond that".
He said: "The legal basis will change because we will no longer be members of the EU. People will no longer be here under the free movement directive.
"They will have arrived as EU citizens within an EU country and after the day we leave they will be here as EU citizens in a third country.
"The exact basis on which they are then entitled to remain in the UK will be determined by the negotiation.
"Does that entail a right to work, a right to seek work, a right to permanent free access to the NHS - all of those things are yet to be determined."
He told the committee the Government has been clear that it does not wish to prejudice the rights of EU citizens who are here "but at the same time we have to protect the rights of British citizens overseas".
Mr Sedwill said there was a distinction between the position of somebody who has permanent residence and others.
"People have got that right of permanent residence and that right is associated with other international treaties that we are members of, such as human rights legislation."
Asked about giving a guarantee, Mr Sedwill said: "I think for people who have got the five-year residence, in effect we have had a guarantee."
He added: "It's not for me to do so. In the end Parliament does. (Under) the current law, that is clearly the case."