National Insurance numbers registered to migrants from eight central and eastern European countries have fallen to the lowest level since the states joined the EU 13 years ago, new analysis shows.
Publishing the findings, researchers said there are indications the UK has become less attractive to nationals from the region since last year's referendum.
A study of official data by the Migration Observatory at the University of Oxford shows a dip in allocations of NI numbers (NINos) to people from so-called A8 countries which became part of the EU in 2004.
The group consists of Poland, Latvia, Lithuania, Estonia, Hungary, Czech Republic, Slovakia and Slovenia.
In the first three months of this year just over 26,000 NINos were registered to migrants from the countries.
This tally was a fall of around a third compared to the first quarter of 2016, and the lowest number of A8 registrations in the equivalent period since the states joined the EU in the middle of 2004.
It was also a fraction of the peak quarterly figure of more than 111,000 recorded in the first three months of 2007.
National Insurance numbers are required by migrants looking to work or claim benefits or tax credits in the UK.
The findings on NINos follow figures showing a fall in net migration from the EU8 countries last year. Immigration from the states was down by 25,000 to 48,000, while emigration increased by 16,000 to 43,000.
These were both statistically significant changes and resulted in the smallest net migration estimate, of 5,000, for the group of nations since they joined the EU.
Dr Carlos Vargas-Silva, acting director of the Migration Observatory, said: "We are seeing indications that the UK has become less of an attractive destination for migrants from Poland and the other A8 countries since the referendum on leaving the EU."
The paper suggested the decline in EU net migration may be linked to the strength of the pound.
It said: "The referendum result led to a wave of concern in global financial markets about the UK economy, which immediately resulted in a decline in the value of the pound against other major currencies - reducing the relative value of wages for foreign workers in the UK."
Dr Vargas-Silva said: "Aside from the economic factors, other things like the lack of clarity about their long-term legal status, and highly-publicised xenophobic attacks, may also have affected EU citizens' choice to come to the UK, or to remain here."
Focus has fallen on the status of more than three million EU nationals in Britain following the Brexit vote, as well as how immigration from the bloc will be managed following the UK's departure.
The report concluded: "The reduction in the net number of EU migrants has occurred even though there has been no actual change in their legal status.
"This suggests that for many EU workers uncertainty related to future residency rights, the value of the pound and the political environment of the UK are likely to play a major role in migration decisions.
"Uncertainty about these three aspects is unlikely to decrease in the near future."
NINo figures differ from the net long-term migration estimates in that they include migrants coming to the country for less than a year, and are based on a recorded registration date so are not a direct measure of when someone arrived in the UK.