Free movement of people will have to be kept for several years after Brexit because a new immigration system will not be in ready in time, a new report has warned.
Curbing migration was a major concern among Leave voters and forms a key part of Theresa May's negotiating priorities yet ministers have been warned it will be "unfeasible" to create a new system by April 2019.
In a new report, the Institute for Government (IfG) think tank said the scale of the administrative challenge was too great and the current immigration system should be kept until a replacement was ready to avoid disruptive changes to labour markets.
It also found the current process for registering European Union (EU) nationals was "not fit for purpose" and the Home Office could require up to 5,000 extra civil servants to cope with large numbers of applications and appeals.
Jill Rutter, IfG Brexit programme director, said: "The political imperative for change in immigration is significant, but so is the administrative challenge.
"The scale of the task - creating a new immigration system - is huge and it is critical that government gets it right.
"The current process for dealing with permanent residence applications from EU nationals is not fit for purpose, as the government itself acknowledged.
"It needs to be streamlined as a matter of urgency and as a first step towards a new post-Brexit system."
Customs checks for EU citizens at the border should be kept to a minimum to assuage fears around the impact of a hard border in Ireland, the report said.
Joe Owen, IfG researcher and report author, said: "Brexit is an opportunity to design an immigration system that is more effective for the country and less burdensome for employers.
"It's important that the Government avoids making multiple changes and introducing unnecessary disruption and confusion.
"To provide stability, we should continue with the existing migration system until the new one is ready."