The UK's biggest mortgage lenders have a 75% market share but offer less than a third of the most competitive deals, research indicated today.
The UK's seven largest lenders, including Government-backed Lloyds Banking Group, Santander and HSBC, account for only 29% of the top 500 best mortgage deals, according to financial information group Moneyfacts.co.uk.
They are least competitive for people with only a small deposit, offering just 12% of the top deals on two-year fixed rate mortgages for people with 15% to put down who are buying a property.
The figure rises only slightly to 18% for a similar borrower remortgaging on to a two-year fixed rate loan, while they offer only 20% of leading rates for someone with a 15% deposit who is remortgaging on to a five-year fixed-rate mortgage.
The big lenders were most competitive on two-year tracker mortgages for borrowers with at least 25% to put down, accounting for 48% of leading deals for house purchase and 44% for those remortgaging.
They also offered 38% of the best rates available to people with a 25% deposit who were looking to remortgage on to a two-year fixed rate loan.
Michelle Slade, spokeswoman for Moneyfacts.co.uk, said: "In many cases, the most competitive mortgage deals are not being offered by the largest mortgage lenders.
"Larger lenders appear to be concentrating on offering more competitive deals to borrowers with at least a 25% deposit."
The group based its figures on the true cost of a £130,000 mortgage, taking into account both interest rates and arrangement fees and other charges.
Ms Slade said: "When selecting a new deal, borrowers should ensure they look at the true cost of the mortgage, including the fee.
"A deal with a low rate but a high fee could work out more expensive than a slightly higher rate with a much lower fee.
"Some of the lowest rates on the market charge a percentage fee, which can work out very expensive if you have a sizeable mortgage."