Financial advisers working at High Street banks have been accused by Which magazine of giving poor advice.
Bad mortgage advice can cost thousands of pounds
Seven of its researchers mystery-shopped 39 financial advisers working for some of the UK's biggest mortgage firms.
The researchers posed as customers and found all but three of the advisers approached offered bad mortgage advice.
Many advisers concentrated more on selling protection insurance than on giving advice, the magazine said.
The researchers alleged that advisers working for High Street names - such as Abbey, Halifax, HSBC, Lloyds TSB, NatWest, Nationwide and Northern Rock - did not make it clear they could recommend only their own companies' mortgages.
In addition, 21 advisers failed to properly explain the ways to repay a mortgage and 23 did not clarify the different deals available, in contravention of the mortgage code.
"Some of the bad advice we received was down to inexperience and poor training, or advisers being more concerned about selling protection insurance that pays big commissions than giving mortgage advice," said Malcolm Coles, editor of Which.
"Bad mortgage advice can cost you thousands of pounds or - even worse - your home."
Halifax Bank, the UK's biggest mortgage lender, told BBC News Online that it took its responsibilities under the mortgage code "very seriously".
"Customers who come into our branches are well aware that they will be offered advice about Halifax products and services," said Mark Hemmingway, Halifax spokesman.
In addition, NatWest and C&G, who undertake mortgage advice in Lloyds TSB branches, said the magazine's findings were not consistent with their own internal research.
NatWest added that it had recently retrained its advisers while C&G said they would closely examine the circumstances of the Which investigation.
Abbey and HSBC told BBC News Online that they were concerned at the magazine's findings and would be looking to investigate.
Nationwide building society said it was very difficult to comment without seeing the full details of the Which report, but they believed that their advisers worked to the highest possible standards.
Northern Rock said they were seeking clarification as to the precise details of the mystery shop.
From November, the mortgage sector will be regulated by the Financial Services Authority (FSA).