The Association of Mortgage Intermediaries (AMI) has lodged a complaint with the Financial Services Authority (FSA) about recent changes some lenders have made to their mortgage ranges, limiting the deals available through intermediaries and raising the cost of the ones that remain.

Increasing numbers of consumers are thought to be turning to brokers for help in negotiating the currently rapidly changing mortgage market. The credit crunch has led to the most competitive mortgage deals being dropped soon after they are launched, sometimes with just hours' notice, as lenders are inundated with business, and consumers taking out a home loan through a broker have a better chance of catching them before they are withdrawn.

But AMI claims lenders' branch-based business is suffering as a result of the high numbers of people choosing to use a broker.

It said margins on deals taken out directly with the lender tended to be higher as they do not have to pay commission to an intermediary.

As a result, a number of Britain's biggest mortgage providers are trying to redress the balance by offering their best deals only to people who contact them directly.

Nationwide Building Society last month launched an offer under which first-time buyers applying for its three-year tracker or three-year fixed rate mortgages could get a £300 discount on their reservation fee and a £100 discount on legal fees, but the deal is only available to customers who go direct.

Halifax plus Cheltenham & Gloucester both recently announced they were increasing their rates by up to 0.6 per cent for people who take out a mortgage through a broker, while keeping the cost of branch-based loans on hold.

The move came just over a week after Halifax had previously increased some of its broker rates by up to 0.5 per cent.

Director general of AMI Chris Cummings has said: "Lenders have seen branch-based business really suffering.

"So in order to try and balance their business, instead of offering equal access to products across all channels, they've started introducing mortgages that are only available direct and not to intermediaries. Lenders are under pressure to rebuild their capital position, and as a result are trying to increase margins and drive more people through their branches.

"But we have taken this up with the FSA and said it is consumers who are losing out."

The group said it understood that lenders had to protect their business models, but it said with about seven out of 10 borrowers taking out their mortgage through a broker, consumers were not getting a good deal. But the FSA indicated it would be unlikely to intervene in the dispute, pointing out that under its regulations, lenders are not obliged to offer mortgages through brokers.

A spokeswoman said: "It is a commercial decision for a lender whether they want to offer products directly to the public or via a broker.

"Anyone dealing with consumers should ensure that consumers are treated fairly."

She added that if certain lenders decided to offer cheaper deals to direct customers, it would not be in consumers' best interests to prevent that.