Immigration is the main factor behind the UK's growing population
Incentives should be offered to keep highly skilled immigrants in the UK, as more are staying for just a short time before leaving, a report argues.
The Institute for Public Policy Research think tank warned the UK could lose what it called "super mobile" migrants to competing countries.
It suggested incentives such as simpler visa extensions and tax breaks.
But campaign group Migrationwatch UK called for immigration limits to help curb population growth.
The centre-left IPPR think tank said that more than 190,000 migrants left the UK in 2007, and the number was likely to have been higher last year.
IPPR said "short-stay migration" was a "growing phenomenon", claiming the number of immigrants spending under four years in the UK doubled between 1996 and 2007.
Those most likely to leave were highly skilled, well educated and with few barriers to movement - the "super mobile" - the IPPR said.
"The migration debate in the UK is fixated with the idea that immigrants come to settle and not enough attention has been paid to the fact that more and more immigrants are spending only short periods in the UK," said the IPPR's head of migration, Tim Finch.
"Our research shows that many groups of migrants are now increasingly mobile. They are coming to the UK to study and work for short periods and then they are moving on.
"As global competition for highly skilled migrants increases in future years, schemes to retain migrants may become as important as attracting them in the first place."
But Migrationwatch UK chairman Sir Andrew Green argued that "re-migration is the least of our problems".
"The real problem is the absence of any effective limits on immigration, often from countries where wages are a fraction of those in the UK and to which people are unlikely to return," he said.
Sir Andrew claimed there was "intense public concern" that the UK population could reach 70 million in the next 20 years.
"Until this is tackled, reports of this kind will fall on very stony ground."
Immigration minister Phil Woolas said: "This report further demonstrates that migrants come to the UK for a short period of time, work, contribute to the economy and then return home."
The points-based system for deciding whether to allow immigration by non-EU citizens was "ensuring that only those that Britain need can come", he said.
Earlier this week, the government unveiled proposals for a points-based system of assessing residents' rights to UK citizenship.
The recommendations of the IPPR report include:
- More active steps to encourage migrants to stay, through the points systems, retention schemes, simplified visa extensions and tax incentives
- Establishment of Migration Information Centres and "circular migration" schemes to manage short-stay migration
- Consideration of the rise in short-stay migration in integration strategies
- Improving links with former immigrants to the UK, identifying them as economic and diplomatic assets.