Consumers shopping around for cheap mortgage deals look set to cost lenders more than £3 billion in the next two years.

This according to a report published yesterday.

The research carried out by PA Consulting said the problem of people leaving their existing lender to take advantage of a lower rate elsewhere, known as churning, would become an increasing challenge.

It estimated that mortgage churn has doubled in the past seven years but two-thirds of lenders are unable to say how much it costs them.

At the same time, three-quarters of lenders say they are most focused on getting new business.

Mark Roberts, one of the authors of the report, said: "We have seen a radical change in consumer behaviour, brought about by discounting, which was unheard of until recently.

"Mortgage churn presents an increasingly expensive challenge."

Churn is expensive because when customers leave, lenders are faced with the costs of acquiring new business, such as advertising and administration, and also have to offer a low mortgage rate in order to win customers.