Northern Rock has tightened its lending criteria, meaning that thousands of existing mortgage borrowers can no longer use the flexible features of their deals.

Northern Rock offered fully flexible mortgages, allowing borrowers to freely overpay money on their homeloans and then borrow it back or take a payment holiday further down the line.

But in the last month, the taxpayer-owned lender has toughened up its affordability criteria.

Ray Boulger, senior technical manager at broker John Charcol, said: "Northern Rock's unique selling point for a number of years has been the flexibility it offered on its mortgages range. If far fewer people can utilise this feature then this is a significant change in its lending policy."

He pointed out that as a result many more people may find they can no longer borrow back overpayments or take payment holidays, even though they may have done so previously.

Boulger said that Northern Rock, along with other lenders, had recently reduced the income multiples it would lend.

Previously customers could have borrowed five times their salary but most banks will now lend just three or four times income.

"In such cases borrowers could find that they can't access these overpayments. Similarly if one or more of the mortgage holders has lost their job, they are also likely to breach affordability criteria."

A spokesman for Northern Rock said: "We have always made clear to borrowers that they have no automatic entitlement to borrow back this money. It is subject to the lender's approval."

But he added that as a "responsible lender" it was in the interest of both the bank and its customers that it lent money only to those who could afford it. As a result it was now asking for additional financial information before granting approval, and so it was likely that more borrowers would fail to get this approval.

"As there has been no change to the terms and conditions, we did not see any reason to write to customers," he added.