Scotland's "Fresh Talent" initiative to entice foreign students to Scotland is "completely unjustified", according to the think tank Migrationwatch UK.
Migrationwatch UK said reports of Scotland's depopulation had been "exaggerated" and said the scheme could increase problems in England. Chairman Sir Andrew Green urged against a separate Scottish immigration policy.
A spokeswoman for the Scottish Executive said the country needed "a constant flow" of fresh talent. First Minister Jack McConnell launched the Fresh Talent initiative in 2004 and said the country needed to attract 8,000 people to come and live in Scotland each year until 2009.
However, Migrationwatch said in its six-page report that immigration to Scotland had been underestimated and it questioned the belief that the country's population would fall to 4.96 million by 2021. It said employers could already hire skilled workers from outside the EU and it claimed separate immigration arrangements for Scotland offered "another back door" to the UK. Sir Andrew said Scotland's population would rise over the next 20 years if recent trends continued.
"The real demographic challenge is population ageing - a problem common to all of the UK, which cannot be addressed effectively by immigration for the simple reason that migrants also age," he said.
Sir Andrew said England already faced "severe challenges" from high population density - six times that of Scotland - and further large rises caused mainly by high immigration levels.
"Proposals for a separate Scottish immigration scheme are completely unjustified by the facts," he said.
"Such a scheme would merely exacerbate the situation in the rest of the UK because applicants granted a work permit on the basis of a shortage in Scotland would, in practice, be able to move south."
The executive said it was "absolutely ridiculous" to suggest Scotland wanted a separate immigration policy. Ministers were promoting specific measures within a UK-wide policy of "managed migration", a spokeswoman said.
"We are working closely with the Home Office to refine and develop initiatives to attract and support new Scots.
"Our aim to attract fresh talent is driven by demographic projections which show Scotland is the only country in the UK whose population is projected to decline and concern that an ageing and shrinking population will affect our economic future."
A Home Office spokesman said the Fresh Talent initiative was a pilot scheme being "closely monitored".