Such organisations held almost 80 per cent of the market share in May, which was an increase from 56 per cent last October.
Mr Kuo stated that the current conditions in the mortgage sector mean that "the once level playing field that borrowers could count on for competitive rates has been tilted in favour of banks".
He added: "If they exploit their privileged position now, then they need to be ready to duck for cover when the pendulum swings back the other way."
Last week, Abbey announced plans to cut fees on its flexible mortgage products.
The lender has reduced the fees on the products, which have rates beginning at 6.43 per cent, from £1,000 to £1,499.
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