Nationwide offers a base mortgage rate of 6.24% to both existing and new borrowers, but GMAC is enticing new customers to a two year fixed rate at 4.99%.
The firm overhauled its lending policies in March, scrapping discount mortgages to new borrowers in order to be fairer to existing customers and bring the overall rate for all borrowers down.
But the strategy - seen as a high-risk one - has not been adopted by other building societies, and borrowers have simply opted for the more attractive headline rate of competing lenders.
Nationwide admits that its sales of mortgages through Independent Financial Advisers have declined, but has not released specific figures.
Struggling to compete
The independent mortgage brokers John Charcol say that the fall in Nationwide mortgage sales is "certainly significant".
A spokeswoman for the brokers told BBC News Online that the firm is supportive of the Nationwide's "well-intentioned" mortgage cuts, but that customers - who always go for the cheapest option - drive demand.
Nationwide offers a base mortgage rate of 6.24% to both existing and new borrowers, but GMAC is enticing new customers to a two year fixed rate at 4.99%.
On a loan of £100,000, this would save the borrower £1,250 (£104/month) in interest.
Nationwide admits that sales of mortgages through intermediaries are declining, but says that it is "happy" with overall sales.
Customer retention
A spokeswoman told BBC News Online that it blames brokers for failing to properly understand the advantages of its new deals, and expects brokers to grasp the "fundamental shift" better in time.
It also says that customer retention, re-mortgaging and direct sales through branches have risen since the changes.
But brokers say that the building society looks likely to continue to lose market share while it is offering less competitive rates.
Non-direct sales used to make up about half of Nationwide's total mortgage sales.
And an increasing number of borrowers are using financial advisers rather than going directly to a bank or building society, given the increasing complexity of the UK mortgage market.
Forced subsidies
In March, Nationwide introduced a new basic rate of 6.49%, called the "base mortgage rate" (BMR), for existing and new borrowers.
Following the Bank of England's interest rate cut, this will now be trimmed to 6.29% from 1 May.
Half a million Nationwide customers have benefited as the rate they pay fell from 7.09% to 6.49% - equivalent to a £23 per month fall on a £60,000 repayment loan.
The building society says that its new rate will protect existing borrowers, who have typically been forced to subsidise new borrowers.
Nationwide estimates there are currently around three million borrowers with mortgages totalling £100bn in the UK.
It says that if all major lenders followed Nationwide's approach it would lead to an overall saving of more than £1bn a year for existing borrowers.