An £18,600 minimum income requirement for British nationals and permanent residents seeking to sponsor a partner from outside Europe to come to the UK was introduced last July.
The Government believes the new rules will save taxpayers £660 million over 10 years through welfare, NHS and education savings - but a study by Middlesex University claims ministers have failed to fully take into account migrants' earnings.
It also argues that preventing a partner from entering the UK is likely to increase welfare claims as single parents are more likely to draw on state support if alone.
Dr Helena Wray, from the School of Law at Middlesex University, who co-authored the research briefing, said: "It appears the Government got its sums wrong when designing this policy.
"When the cost-benefit calculations for this policy in the impact assessment are properly carried out, the figures actually show that the income requirement could cost the public purse £850 million over 10 years.
But the Home Office rejected the report's findings. It said the £850 million figure referred to in the report was "incorrect" as it included the taxes paid by migrant partners, but not their costs to public services, and therefore overestimated the positive effect of family migrants.
"We do not accept the conclusions reached by Middlesex University," a Home Office spokesman said. "We estimate that the minimum income threshold, set following advice from the independent Migration Advisory Committee, will benefit the taxpayer by £660 million over 10 years.
"This includes both the expected savings in public services and the reduction in taxes paid by migrant spouses and partners".