Abbey will introduce a tougher affordability test for new mortgage customers tomorrow.

Britain's second biggest lender is increasing the interest rate that it uses to work out whether new borrowers can afford a mortgage. Abbey will now base its calculations on a higher rate of 7 per cent.

Most lenders use their standard variable rate (SVR) as a basis to calculate whether someone can afford monthly mortgage repayments.

However, SVRs have fallen sharply in the last two months after lenders were put under heavy political pressure to pass on the Bank of England base rate cut to borrowers on variable-rate deals. Abbey's SVR is currently 5.44 per cent.

The lender will also use only a capital repayment mortgage as the basis for its calculations, rather than interest-only, which would make monthly repayments cheaper.

Aaron Strutt, of Chase De Vere Mortgage Management, a broker, said: "Abbey are clearly predicting that when people come off their two- or three-year deals their SVR is going to be as high as 7 per cent. However new customers taking out a mortgage now may think it unfair that the rate being used is far higher than the current SVR or the base rate".

Lenders are paying even closer attention to mortgage applications and the credit histories of new customers, who face being rejected for being overdrawn on their current account or missing a mobile phone payment.

Melanie Bien, director of Savills Private Finance, another broker, said: "There is a danger that lenders become too cautious, penalising all borrowers for some of the lax lending policies of the past.

"Applications really should be assessed on a case-by-case basis to ensure that prime borrowers are not penalised for problems others have had in paying their mortgage in the past."

Abbey came under fire earlier this week for writing to some borrowers on flexible mortgages, warning them of a clause in their contract which could force them to pay back a lump sum if their home falls too far in value.

Flexible mortgage customers can extend the amount of money they have borrowed against their home, up to 90 per cent of the property's value.

However, Abbey has warned borrowers that if the value of their home continues to fall, pushing up their loan-to-value (LTV) ratio beyond 90 per cent, households may be forced to stump up a lump sum to ensure it is reduced back to below 90 per cent.

A spokesperson for Abbey said: "We have not invoked this particular clause and have no intention to. What we would say is that in a falling house price environment it is prudent for people on flexible deals who find themselves at over 90 per cent LTV to look at whether they can afford to make any overpayments."

House prices are predicted to fall by 15 per cent over the next 12 months.