New laws designed to slash the number of migrants by 300,000 a year risk splitting up families already living in the UK.
Brits could see their foreign partners told to leave the country the next time their visa comes up for renewal – if their household does not earn £38,700, No 10 said.
Experts, however, warned the planned crackdown was causing distress for many.
Downing Street defended the policy, saying it was right that “if you are bringing someone into the country you are able to support them”.
Under the plans unveiled on Monday those wishing to bring their spouse to the UK will now have to earn £38,700, a significant increase on the current figure of £18,600, and what has been described as a tax on love.
Former Tory minister Gavin Barwell said it was “both morally wrong and unconservative to say that only the wealthiest can fall in love, marry someone and then bring them to the UK”.
As well as applying to those yet to come to the UK, No 10 confirmed the new higher figure risks affecting those already here.
Asked if it would apply to partners when they came to renew their visas, No 10 said the change was “not retrospective, but it would apply to renewals in the future”.
At that point, people would be expected to “meet the visa requirements of the day”.
The prime minister’s official spokesperson added: “People always have a set length of time for their visas and will be aware at the conclusion of that visa time that they don’t have a guarantee that they will obviously remain in the country.”
The income level had not been updated for 10 years, No 10 said.
There are exemptions, but these will be on a case-by-case basis, and it stressed that the amount could be spread across the household.
Madeleine Sumption, the director of the Migration Observatory at the University of Oxford, said the changes would make the UK an outlier among western nations.
“This is definitely completely different to what any other high-income country does,” she said.
Josephine Whitaker-Yilmaz, from migrant rights organisation Praxis, said she was concerned for families who feel “forced to make horrible decisions” due to the changes.
The changes are due to come into effect in spring and will apply when existing visas for foreign spouses come up for renewal.
The prime minister’s spokesperson said on Tuesday: “The family immigration rules contain a provision for exceptional circumstances where there would be unjustifiably harsh consequences for the applicant, their partner, a relevant child or another family member if their application were to be refused.”
No examples were given and the spokesperson said applications would be considered on a “case-by-case basis”.
Official figures from April show the median gross annual earnings for full-time employees in the UK was £34,963.