Brokers generally recommend you overpay your mortgage to clear as much debt as possible and make sure you qualify for the best deals.

For example, a borrower with a £200,000 loan on a £300,000 property who overpaid by 4.5% (the size of Bank rate cuts since October) or £750 a month would reduce their mortgage to £182,000 over two years, cutting their loan as a proportion of their property value from 67% to about 60%. This would give them access to cheaper deals, according to L&C Mortgages.

However, Northern Rock customers who overpaid on the basis that they would be able to withdraw the money again if they needed to — as is common with “flexible” mortgages — have been warned this may no longer be an option.

One Sunday Times reader who had overpaid £1m on his £3m loan faced a barrage of questions about his income and outgoings when he applied to withdraw £25,000. His request was turned down by the call centre staff and his case referred to the underwriter.

Ian Gray, of independent mortgage broker Largemortgageloans.co.uk, said: “This is yet another example of lenders tightening criteria for borrowers unexpectedly. Many people took out those flexible deals with the promise that it was a place to park cash and they could get it next day with a phone call. There are now serious questions about whether they can get their money.”

Borrowers on flexible, standard or offset mortgages should be able to overpay without penalty. Other deals may limit payments to 10% of the loan.

Brokers also advise those looking to protect themselves from falling property values to consider booking a deal in advance. Nationwide allows this three months in advance without another valuation.