Borrowers on high incomes could see their ability to take out mortgage loans limited next year, following the introduction of a higher rate of income tax.

Most banks now calculate how much they are willing to lend according to customers’ disposable income, rather than a multiple of their gross earnings. This means anyone who will see their net income reduce next year as a result of the new higher tax rate could find their borrowing power curbed. Brokers said some lenders could reduce borrowing amounts by 10 per cent to reflect the increase in tax from 40 to 50 per cent.

“We think the new 50p rate will hinder the borrowing capacity of those on higher incomes,” said Ian Gray, a broker at Largemortgageloans.com. “Banks now factor in things like monthly outgoings and dependants, but, more importantly, income tax and National Insurance deductions to determine what kind of mortgage payment is affordable from net income.”

He calculates that applicants earning £150,000 with no other debts and a 25 per cent deposit could expect to borrow around £850,000. But if their net income fell by 10 per cent, they could find their maximum loan size reduced to around £760,000.

Banks have generally stopped lending on strict income multiples to give a fairer reflection of the affordability of the loan. This means someone with a lower salary but minimal out- goings could qualify for a larger loan. Gray said Northern Rock still used traditional income multiples so its mortgages will be unaffected by higher tax.