As a result, 3.75 million families will be forced to pay a collective £8bn extra in the coming years when they come to remortgage at higher rates, the Resolution Foundation said.
Around 1.6 million homeowners will see an average annual increase of £2,300 in their mortgage bills over the next 12 months.
The new affordability checks put the Financial Conduct Authority further at odds with the Treasury, which wants to reduce bureaucracy in the financial sector.
The watchdog has previously attracted criticism for being slow to authorise new businesses.
Borrowers can normally choose to transfer to a new product with their current lender when remortgaging without having to undergo any further box-ticking.
But banks will now have to assess existing customers if they believe rising interest rates are “material” to their affordability in times of financial turmoil, such as that seen in the wake of the mini-Budget.
The added bureaucracy comes despite Chancellor Jeremy Hunt in December urging banks to be as “flexible” as possible as borrowers face soaring interest rates and the worst cost of living crisis in decades.
Lobby group UK Finance has previously warned the watchdog against implementing the new rules.
In a stern letter, it urged the FCA to drop its plans to impose the extra checks late last year.
“Declining borrowers who are up to date with their payments and seeking to switch their mortgage will create a further cohort of mortgage prisoners,” UK Finance said.
“Requiring an affordability test in this instance has the potential for greater harm, as borrowers will not be able to protect themselves from further increases to variable rates.”
